src/net/smtp/smtp.go - go - Git at Google

 // Copyright 2010 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // Package smtp implements the Simple Mail Transfer Protocol as defined in RFC 5321.
 // It also implements the following extensions:
 //
 //	8BITMIME  RFC 1652
 //	AUTH      RFC 2554
 //	STARTTLS  RFC 3207
 //
 // Additional extensions may be handled by clients.
 //
 // The smtp package is frozen and is not accepting new features.
 // Some external packages provide more functionality. See:
 //
 //	https://godoc.org/?q=smtp
 package smtp

 import (
 	"crypto/tls"
 	"encoding/base64"
 	"errors"
 	"fmt"
 	"io"
 	"net"
 	"net/textproto"
 	"strings"
 )

 // A Client represents a client connection to an SMTP server.
 type Client struct {
 	// Text is the textproto.Conn used by the Client. It is exported to allow for
 	// clients to add extensions.
 	Text *textproto.Conn
 	// keep a reference to the connection so it can be used to create a TLS
 	// connection later
 	conn net.Conn
 	// whether the Client is using TLS
 	tls        bool
 	serverName string
 	// map of supported extensions
 	ext map[string]string
 	// supported auth mechanisms
 	auth       []string
 	localName  string // the name to use in HELO/EHLO
 	didHello   bool   // whether we've said HELO/EHLO
 	helloError error  // the error from the hello
 }

 // Dial returns a new [Client] connected to an SMTP server at addr.
 // The addr must include a port, as in "mail.example.com:smtp".
 func Dial(addr string) (*Client, error) {
 	conn, err := net.Dial("tcp", addr)
 	if err != nil {
 		return nil, err
 	}
 	host, _, _ := net.SplitHostPort(addr)
 	return NewClient(conn, host)
 }

 // NewClient returns a new [Client] using an existing connection and host as a
 // server name to be used when authenticating.
 func NewClient(conn net.Conn, host string) (*Client, error) {
 	text := textproto.NewConn(conn)
 	_, _, err := text.ReadResponse(220)
 	if err != nil {
 		text.Close()
 		return nil, err
 	}
 	c := &Client{Text: text, conn: conn, serverName: host, localName: "localhost"}
 	_, c.tls = conn.(*tls.Conn)
 	return c, nil
 }

 // Close closes the connection.
 func (c *Client) Close() error {
 	return c.Text.Close()
 }

 // hello runs a hello exchange if needed.
 func (c *Client) hello() error {
 	if !c.didHello {
 		c.didHello = true
 		err := c.ehlo()
 		if err != nil {
 			c.helloError = c.helo()
 		}
 	}
 	return c.helloError
 }

 // Hello sends a HELO or EHLO to the server as the given host name.
 // Calling this method is only necessary if the client needs control
 // over the host name used. The client will introduce itself as "localhost"
 // automatically otherwise. If Hello is called, it must be called before
 // any of the other methods.
 func (c *Client) Hello(localName string) error {
 	if err := validateLine(localName); err != nil {
 		return err
 	}
 	if c.didHello {
 		return errors.New("smtp: Hello called after other methods")
 	}
 	c.localName = localName
 	return c.hello()
 }

 // cmd is a convenience function that sends a command and returns the response
 func (c *Client) cmd(expectCode int, format string, args ...any) (int, string, error) {
 	id, err := c.Text.Cmd(format, args...)
 	if err != nil {
 		return 0, "", err
 	}
 	c.Text.StartResponse(id)
 	defer c.Text.EndResponse(id)
 	code, msg, err := c.Text.ReadResponse(expectCode)
 	return code, msg, err
 }

 // helo sends the HELO greeting to the server. It should be used only when the
 // server does not support ehlo.
 func (c *Client) helo() error {
 	c.ext = nil
 	_, _, err := c.cmd(250, "HELO %s", c.localName)
 	return err
 }

 // ehlo sends the EHLO (extended hello) greeting to the server. It
 // should be the preferred greeting for servers that support it.
 func (c *Client) ehlo() error {
 	_, msg, err := c.cmd(250, "EHLO %s", c.localName)
 	if err != nil {
 		return err
 	}
 	ext := make(map[string]string)
 	extList := strings.Split(msg, "\n")
 	if len(extList) > 1 {
 		extList = extList[1:]
 		for _, line := range extList {
 			k, v, _ := strings.Cut(line, " ")
 			ext[k] = v
 		}
 	}
 	if mechs, ok := ext["AUTH"]; ok {
 		c.auth = strings.Split(mechs, " ")
 	}
 	c.ext = ext
 	return err
 }

 // StartTLS sends the STARTTLS command and encrypts all further communication.
 // Only servers that advertise the STARTTLS extension support this function.
 func (c *Client) StartTLS(config *tls.Config) error {
 	if err := c.hello(); err != nil {
 		return err
 	}
 	_, _, err := c.cmd(220, "STARTTLS")
 	if err != nil {
 		return err
 	}
 	c.conn = tls.Client(c.conn, config)
 	c.Text = textproto.NewConn(c.conn)
 	c.tls = true
 	return c.ehlo()
 }

 // TLSConnectionState returns the client's TLS connection state.
 // The return values are their zero values if [Client.StartTLS] did
 // not succeed.
 func (c *Client) TLSConnectionState() (state tls.ConnectionState, ok bool) {
 	tc, ok := c.conn.(*tls.Conn)
 	if !ok {
 		return
 	}
 	return tc.ConnectionState(), true
 }

 // Verify checks the validity of an email address on the server.
 // If Verify returns nil, the address is valid. A non-nil return
 // does not necessarily indicate an invalid address. Many servers
 // will not verify addresses for security reasons.
 func (c *Client) Verify(addr string) error {
 	if err := validateLine(addr); err != nil {
 		return err
 	}
 	if err := c.hello(); err != nil {
 		return err
 	}
 	_, _, err := c.cmd(250, "VRFY %s", addr)
 	return err
 }

 // Auth authenticates a client using the provided authentication mechanism.
 // A failed authentication closes the connection.
 // Only servers that advertise the AUTH extension support this function.
 func (c *Client) Auth(a Auth) error {
 	if err := c.hello(); err != nil {
 		return err
 	}
 	encoding := base64.StdEncoding
 	mech, resp, err := a.Start(&ServerInfo{c.serverName, c.tls, c.auth})
 	if err != nil {
 		c.Quit()
 		return err
 	}
 	resp64 := make([]byte, encoding.EncodedLen(len(resp)))
 	encoding.Encode(resp64, resp)
 	code, msg64, err := c.cmd(0, strings.TrimSpace(fmt.Sprintf("AUTH %s %s", mech, resp64)))
 	for err == nil {
 		var msg []byte
 		switch code {
 		case 334:
 			msg, err = encoding.DecodeString(msg64)
 		case 235:
 			// the last message isn't base64 because it isn't a challenge
 			msg = []byte(msg64)
 		default:
 			err = &textproto.Error{Code: code, Msg: msg64}
 		}
 		if err == nil {
 			resp, err = a.Next(msg, code == 334)
 		}
 		if err != nil {
 			// abort the AUTH
 			c.cmd(501, "*")
 			c.Quit()
 			break
 		}
 		if resp == nil {
 			break
 		}
 		resp64 = make([]byte, encoding.EncodedLen(len(resp)))
 		encoding.Encode(resp64, resp)
 		code, msg64, err = c.cmd(0, string(resp64))
 	}
 	return err
 }

 // Mail issues a MAIL command to the server using the provided email address.
 // If the server supports the 8BITMIME extension, Mail adds the BODY=8BITMIME
 // parameter. If the server supports the SMTPUTF8 extension, Mail adds the
 // SMTPUTF8 parameter.
 // This initiates a mail transaction and is followed by one or more [Client.Rcpt] calls.
 func (c *Client) Mail(from string) error {
 	if err := validateLine(from); err != nil {
 		return err
 	}
 	if err := c.hello(); err != nil {
 		return err
 	}
 	cmdStr := "MAIL FROM:<%s>"
 	if c.ext != nil {
 		if _, ok := c.ext["8BITMIME"]; ok {
 			cmdStr += " BODY=8BITMIME"
 		}
 		if _, ok := c.ext["SMTPUTF8"]; ok {
 			cmdStr += " SMTPUTF8"
 		}
 	}
 	_, _, err := c.cmd(250, cmdStr, from)
 	return err
 }

 // Rcpt issues a RCPT command to the server using the provided email address.
 // A call to Rcpt must be preceded by a call to [Client.Mail] and may be followed by
 // a [Client.Data] call or another Rcpt call.
 func (c *Client) Rcpt(to string) error {
 	if err := validateLine(to); err != nil {
 		return err
 	}
 	_, _, err := c.cmd(25, "RCPT TO:<%s>", to)
 	return err
 }

 type dataCloser struct {
 	c *Client
 	io.WriteCloser
 }

 func (d *dataCloser) Close() error {
 	d.WriteCloser.Close()
 	_, _, err := d.c.Text.ReadResponse(250)
 	return err
 }

 // Data issues a DATA command to the server and returns a writer that
 // can be used to write the mail headers and body. The caller should
 // close the writer before calling any more methods on c. A call to
 // Data must be preceded by one or more calls to [Client.Rcpt].
 func (c *Client) Data() (io.WriteCloser, error) {
 	_, _, err := c.cmd(354, "DATA")
 	if err != nil {
 		return nil, err
 	}
 	return &dataCloser{c, c.Text.DotWriter()}, nil
 }

 var testHookStartTLS func(*tls.Config) // nil, except for tests

 // SendMail connects to the server at addr, switches to TLS if
 // possible, authenticates with the optional mechanism a if possible,
 // and then sends an email from address from, to addresses to, with
 // message msg.
 // The addr must include a port, as in "mail.example.com:smtp".
 //
 // The addresses in the to parameter are the SMTP RCPT addresses.
 //
 // The msg parameter should be an RFC 822-style email with headers
 // first, a blank line, and then the message body. The lines of msg
 // should be CRLF terminated. The msg headers should usually include
 // fields such as "From", "To", "Subject", and "Cc".  Sending "Bcc"
 // messages is accomplished by including an email address in the to
 // parameter but not including it in the msg headers.
 //
 // The SendMail function and the net/smtp package are low-level
 // mechanisms and provide no support for DKIM signing, MIME
 // attachments (see the mime/multipart package), or other mail
 // functionality. Higher-level packages exist outside of the standard
 // library.
 func SendMail(addr string, a Auth, from string, to []string, msg []byte) error {
 	if err := validateLine(from); err != nil {
 		return err
 	}
 	for _, recp := range to {
 		if err := validateLine(recp); err != nil {
 			return err
 		}
 	}
 	c, err := Dial(addr)
 	if err != nil {
 		return err
 	}
 	defer c.Close()
 	if err = c.hello(); err != nil {
 		return err
 	}
 	if ok, _ := c.Extension("STARTTLS"); ok {
 		config := &tls.Config{ServerName: c.serverName}
 		if testHookStartTLS != nil {
 			testHookStartTLS(config)
 		}
 		if err = c.StartTLS(config); err != nil {
 			return err
 		}
 	}
 	if a != nil && c.ext != nil {
 		if _, ok := c.ext["AUTH"]; !ok {
 			return errors.New("smtp: server doesn't support AUTH")
 		}
 		if err = c.Auth(a); err != nil {
 			return err
 		}
 	}
 	if err = c.Mail(from); err != nil {
 		return err
 	}
 	for _, addr := range to {
 		if err = c.Rcpt(addr); err != nil {
 			return err
 		}
 	}
 	w, err := c.Data()
 	if err != nil {
 		return err
 	}
 	_, err = w.Write(msg)
 	if err != nil {
 		return err
 	}
 	err = w.Close()
 	if err != nil {
 		return err
 	}
 	return c.Quit()
 }

 // Extension reports whether an extension is support by the server.
 // The extension name is case-insensitive. If the extension is supported,
 // Extension also returns a string that contains any parameters the
 // server specifies for the extension.
 func (c *Client) Extension(ext string) (bool, string) {
 	if err := c.hello(); err != nil {
 		return false, ""
 	}
 	if c.ext == nil {
 		return false, ""
 	}
 	ext = strings.ToUpper(ext)
 	param, ok := c.ext[ext]
 	return ok, param
 }

 // Reset sends the RSET command to the server, aborting the current mail
 // transaction.
 func (c *Client) Reset() error {
 	if err := c.hello(); err != nil {
 		return err
 	}
 	_, _, err := c.cmd(250, "RSET")
 	return err
 }

 // Noop sends the NOOP command to the server. It does nothing but check
 // that the connection to the server is okay.
 func (c *Client) Noop() error {
 	if err := c.hello(); err != nil {
 		return err
 	}
 	_, _, err := c.cmd(250, "NOOP")
 	return err
 }

 // Quit sends the QUIT command and closes the connection to the server.
 func (c *Client) Quit() error {
 	if err := c.hello(); err != nil {
 		return err
 	}
 	_, _, err := c.cmd(221, "QUIT")
 	if err != nil {
 		return err
 	}
 	return c.Text.Close()
 }

 // validateLine checks to see if a line has CR or LF as per RFC 5321.
 func validateLine(line string) error {
 	if strings.ContainsAny(line, "\n\r") {
 		return errors.New("smtp: A line must not contain CR or LF")
 	}
 	return nil
 }
	// Copyright 2010 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// Package smtp implements the Simple Mail Transfer Protocol as defined in RFC 5321.
	// It also implements the following extensions:
	//
	// 8BITMIME RFC 1652
	// AUTH RFC 2554
	// STARTTLS RFC 3207
	//
	// Additional extensions may be handled by clients.
	//
	// The smtp package is frozen and is not accepting new features.
	// Some external packages provide more functionality. See:
	//
	// https://godoc.org/?q=smtp
	package smtp

	import (
	"crypto/tls"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"net"
	"net/textproto"
	"strings"
	)

	// A Client represents a client connection to an SMTP server.
	type Client struct {
	// Text is the textproto.Conn used by the Client. It is exported to allow for
	// clients to add extensions.
	Text *textproto.Conn
	// keep a reference to the connection so it can be used to create a TLS
	// connection later
	conn net.Conn
	// whether the Client is using TLS
	tls bool
	serverName string
	// map of supported extensions
	ext map[string]string
	// supported auth mechanisms
	auth []string
	localName string // the name to use in HELO/EHLO
	didHello bool // whether we've said HELO/EHLO
	helloError error // the error from the hello
	}

	// Dial returns a new [Client] connected to an SMTP server at addr.
	// The addr must include a port, as in "mail.example.com:smtp".
	func Dial(addr string) (*Client, error) {
	conn, err := net.Dial("tcp", addr)
	if err != nil {
	return nil, err
	}
	host, _, _ := net.SplitHostPort(addr)
	return NewClient(conn, host)
	}

	// NewClient returns a new [Client] using an existing connection and host as a
	// server name to be used when authenticating.
	func NewClient(conn net.Conn, host string) (*Client, error) {
	text := textproto.NewConn(conn)
	_, _, err := text.ReadResponse(220)
	if err != nil {
	text.Close()
	return nil, err
	}
	c := &Client{Text: text, conn: conn, serverName: host, localName: "localhost"}
	_, c.tls = conn.(*tls.Conn)
	return c, nil
	}

	// Close closes the connection.
	func (c *Client) Close() error {
	return c.Text.Close()
	}

	// hello runs a hello exchange if needed.
	func (c *Client) hello() error {
	if !c.didHello {
	c.didHello = true
	err := c.ehlo()
	if err != nil {
	c.helloError = c.helo()
	}
	}
	return c.helloError
	}

	// Hello sends a HELO or EHLO to the server as the given host name.
	// Calling this method is only necessary if the client needs control
	// over the host name used. The client will introduce itself as "localhost"
	// automatically otherwise. If Hello is called, it must be called before
	// any of the other methods.
	func (c *Client) Hello(localName string) error {
	if err := validateLine(localName); err != nil {
	return err
	}
	if c.didHello {
	return errors.New("smtp: Hello called after other methods")
	}
	c.localName = localName
	return c.hello()
	}

	// cmd is a convenience function that sends a command and returns the response
	func (c *Client) cmd(expectCode int, format string, args ...any) (int, string, error) {
	id, err := c.Text.Cmd(format, args...)
	if err != nil {
	return 0, "", err
	}
	c.Text.StartResponse(id)
	defer c.Text.EndResponse(id)
	code, msg, err := c.Text.ReadResponse(expectCode)
	return code, msg, err
	}

	// helo sends the HELO greeting to the server. It should be used only when the
	// server does not support ehlo.
	func (c *Client) helo() error {
	c.ext = nil
	_, _, err := c.cmd(250, "HELO %s", c.localName)
	return err
	}

	// ehlo sends the EHLO (extended hello) greeting to the server. It
	// should be the preferred greeting for servers that support it.
	func (c *Client) ehlo() error {
	_, msg, err := c.cmd(250, "EHLO %s", c.localName)
	if err != nil {
	return err
	}
	ext := make(map[string]string)
	extList := strings.Split(msg, "\n")
	if len(extList) > 1 {
	extList = extList[1:]
	for _, line := range extList {
	k, v, _ := strings.Cut(line, " ")
	ext[k] = v
	}
	}
	if mechs, ok := ext["AUTH"]; ok {
	c.auth = strings.Split(mechs, " ")
	}
	c.ext = ext
	return err
	}

	// StartTLS sends the STARTTLS command and encrypts all further communication.
	// Only servers that advertise the STARTTLS extension support this function.
	func (c Client) StartTLS(config tls.Config) error {
	if err := c.hello(); err != nil {
	return err
	}
	_, _, err := c.cmd(220, "STARTTLS")
	if err != nil {
	return err
	}
	c.conn = tls.Client(c.conn, config)
	c.Text = textproto.NewConn(c.conn)
	c.tls = true
	return c.ehlo()
	}

	// TLSConnectionState returns the client's TLS connection state.
	// The return values are their zero values if [Client.StartTLS] did
	// not succeed.
	func (c *Client) TLSConnectionState() (state tls.ConnectionState, ok bool) {
	tc, ok := c.conn.(*tls.Conn)
	if !ok {
	return
	}
	return tc.ConnectionState(), true
	}

	// Verify checks the validity of an email address on the server.
	// If Verify returns nil, the address is valid. A non-nil return
	// does not necessarily indicate an invalid address. Many servers
	// will not verify addresses for security reasons.
	func (c *Client) Verify(addr string) error {
	if err := validateLine(addr); err != nil {
	return err
	}
	if err := c.hello(); err != nil {
	return err
	}
	_, _, err := c.cmd(250, "VRFY %s", addr)
	return err
	}

	// Auth authenticates a client using the provided authentication mechanism.
	// A failed authentication closes the connection.
	// Only servers that advertise the AUTH extension support this function.
	func (c *Client) Auth(a Auth) error {
	if err := c.hello(); err != nil {
	return err
	}
	encoding := base64.StdEncoding
	mech, resp, err := a.Start(&ServerInfo{c.serverName, c.tls, c.auth})
	if err != nil {
	c.Quit()
	return err
	}
	resp64 := make([]byte, encoding.EncodedLen(len(resp)))
	encoding.Encode(resp64, resp)
	code, msg64, err := c.cmd(0, strings.TrimSpace(fmt.Sprintf("AUTH %s %s", mech, resp64)))
	for err == nil {
	var msg []byte
	switch code {
	case 334:
	msg, err = encoding.DecodeString(msg64)
	case 235:
	// the last message isn't base64 because it isn't a challenge
	msg = []byte(msg64)
	default:
	err = &textproto.Error{Code: code, Msg: msg64}
	}
	if err == nil {
	resp, err = a.Next(msg, code == 334)
	}
	if err != nil {
	// abort the AUTH
	c.cmd(501, "*")
	c.Quit()
	break
	}
	if resp == nil {
	break
	}
	resp64 = make([]byte, encoding.EncodedLen(len(resp)))
	encoding.Encode(resp64, resp)
	code, msg64, err = c.cmd(0, string(resp64))
	}
	return err
	}

	// Mail issues a MAIL command to the server using the provided email address.
	// If the server supports the 8BITMIME extension, Mail adds the BODY=8BITMIME
	// parameter. If the server supports the SMTPUTF8 extension, Mail adds the
	// SMTPUTF8 parameter.
	// This initiates a mail transaction and is followed by one or more [Client.Rcpt] calls.
	func (c *Client) Mail(from string) error {
	if err := validateLine(from); err != nil {
	return err
	}
	if err := c.hello(); err != nil {
	return err
	}
	cmdStr := "MAIL FROM:<%s>"
	if c.ext != nil {
	if _, ok := c.ext["8BITMIME"]; ok {
	cmdStr += " BODY=8BITMIME"
	}
	if _, ok := c.ext["SMTPUTF8"]; ok {
	cmdStr += " SMTPUTF8"
	}
	}
	_, _, err := c.cmd(250, cmdStr, from)
	return err
	}

	// Rcpt issues a RCPT command to the server using the provided email address.
	// A call to Rcpt must be preceded by a call to [Client.Mail] and may be followed by
	// a [Client.Data] call or another Rcpt call.
	func (c *Client) Rcpt(to string) error {
	if err := validateLine(to); err != nil {
	return err
	}
	_, _, err := c.cmd(25, "RCPT TO:<%s>", to)
	return err
	}

	type dataCloser struct {
	c *Client
	io.WriteCloser
	}

	func (d *dataCloser) Close() error {
	d.WriteCloser.Close()
	_, _, err := d.c.Text.ReadResponse(250)
	return err
	}

	// Data issues a DATA command to the server and returns a writer that
	// can be used to write the mail headers and body. The caller should
	// close the writer before calling any more methods on c. A call to
	// Data must be preceded by one or more calls to [Client.Rcpt].
	func (c *Client) Data() (io.WriteCloser, error) {
	_, _, err := c.cmd(354, "DATA")
	if err != nil {
	return nil, err
	}
	return &dataCloser{c, c.Text.DotWriter()}, nil
	}

	var testHookStartTLS func(*tls.Config) // nil, except for tests

	// SendMail connects to the server at addr, switches to TLS if
	// possible, authenticates with the optional mechanism a if possible,
	// and then sends an email from address from, to addresses to, with
	// message msg.
	// The addr must include a port, as in "mail.example.com:smtp".
	//
	// The addresses in the to parameter are the SMTP RCPT addresses.
	//
	// The msg parameter should be an RFC 822-style email with headers
	// first, a blank line, and then the message body. The lines of msg
	// should be CRLF terminated. The msg headers should usually include
	// fields such as "From", "To", "Subject", and "Cc". Sending "Bcc"
	// messages is accomplished by including an email address in the to
	// parameter but not including it in the msg headers.
	//
	// The SendMail function and the net/smtp package are low-level
	// mechanisms and provide no support for DKIM signing, MIME
	// attachments (see the mime/multipart package), or other mail
	// functionality. Higher-level packages exist outside of the standard
	// library.
	func SendMail(addr string, a Auth, from string, to []string, msg []byte) error {
	if err := validateLine(from); err != nil {
	return err
	}
	for _, recp := range to {
	if err := validateLine(recp); err != nil {
	return err
	}
	}
	c, err := Dial(addr)
	if err != nil {
	return err
	}
	defer c.Close()
	if err = c.hello(); err != nil {
	return err
	}
	if ok, _ := c.Extension("STARTTLS"); ok {
	config := &tls.Config{ServerName: c.serverName}
	if testHookStartTLS != nil {
	testHookStartTLS(config)
	}
	if err = c.StartTLS(config); err != nil {
	return err
	}
	}
	if a != nil && c.ext != nil {
	if _, ok := c.ext["AUTH"]; !ok {
	return errors.New("smtp: server doesn't support AUTH")
	}
	if err = c.Auth(a); err != nil {
	return err
	}
	}
	if err = c.Mail(from); err != nil {
	return err
	}
	for _, addr := range to {
	if err = c.Rcpt(addr); err != nil {
	return err
	}
	}
	w, err := c.Data()
	if err != nil {
	return err
	}
	_, err = w.Write(msg)
	if err != nil {
	return err
	}
	err = w.Close()
	if err != nil {
	return err
	}
	return c.Quit()
	}

	// Extension reports whether an extension is support by the server.
	// The extension name is case-insensitive. If the extension is supported,
	// Extension also returns a string that contains any parameters the
	// server specifies for the extension.
	func (c *Client) Extension(ext string) (bool, string) {
	if err := c.hello(); err != nil {
	return false, ""
	}
	if c.ext == nil {
	return false, ""
	}
	ext = strings.ToUpper(ext)
	param, ok := c.ext[ext]
	return ok, param
	}

	// Reset sends the RSET command to the server, aborting the current mail
	// transaction.
	func (c *Client) Reset() error {
	if err := c.hello(); err != nil {
	return err
	}
	_, _, err := c.cmd(250, "RSET")
	return err
	}

	// Noop sends the NOOP command to the server. It does nothing but check
	// that the connection to the server is okay.
	func (c *Client) Noop() error {
	if err := c.hello(); err != nil {
	return err
	}
	_, _, err := c.cmd(250, "NOOP")
	return err
	}

	// Quit sends the QUIT command and closes the connection to the server.
	func (c *Client) Quit() error {
	if err := c.hello(); err != nil {
	return err
	}
	_, _, err := c.cmd(221, "QUIT")
	if err != nil {
	return err
	}
	return c.Text.Close()
	}

	// validateLine checks to see if a line has CR or LF as per RFC 5321.
	func validateLine(line string) error {
	if strings.ContainsAny(line, "\n\r") {
	return errors.New("smtp: A line must not contain CR or LF")
	}
	return nil
	}