diff --git a/git-codereview/mail.go b/git-codereview/mail.go
index d6c1108..c6eee7e 100644
--- a/git-codereview/mail.go
+++ b/git-codereview/mail.go
@@ -8,6 +8,7 @@
 	"fmt"
 	"os"
 	"regexp"
+	"sort"
 	"strings"
 )
 
@@ -80,25 +81,116 @@
 
 // mailAddressRE matches the mail addresses we admit. It's restrictive but admits
 // all the addresses in the Go CONTRIBUTORS file at time of writing (tested separately).
-var mailAddressRE = regexp.MustCompile(`^[a-zA-Z0-9][-_.a-zA-Z0-9]*@[-_.a-zA-Z0-9]+$`)
+var mailAddressRE = regexp.MustCompile(`^([a-zA-Z0-9][-_.a-zA-Z0-9]*)(@[-_.a-zA-Z0-9]+)?$`)
 
 // mailList turns the list of mail addresses from the flag value into the format
 // expected by gerrit. The start argument is a % or , depending on where we
 // are in the processing sequence.
 func mailList(start, tag string, flagList string) string {
+	errors := false
 	spec := start
+	short := ""
+	long := ""
 	for i, addr := range strings.Split(flagList, ",") {
-		if !mailAddressRE.MatchString(addr) {
-			dief("%q is not a valid reviewer mail address", addr)
+		m := mailAddressRE.FindStringSubmatch(addr)
+		if m == nil {
+			printf("invalid reviewer mail address: %s", addr)
+			errors = true
+			continue
+		}
+		if m[2] == "" {
+			email := mailLookup(addr)
+			if email == "" {
+				printf("unknown reviewer: %s", addr)
+				errors = true
+				continue
+			}
+			short += "," + addr
+			long += "," + email
+			addr = email
 		}
 		if i > 0 {
 			spec += ","
 		}
 		spec += tag + "=" + addr
 	}
+	if short != "" {
+		verbosef("expanded %s to %s", short[1:], long[1:])
+	}
+	if errors {
+		die()
+	}
 	return spec
 }
 
+// reviewers is the list of reviewers for the current repository,
+// sorted by how many reviews each has done.
+var reviewers []reviewer
+
+type reviewer struct {
+	addr  string
+	count int
+}
+
+// mailLookup translates the short name (like adg) into a full
+// email address (like adg@golang.org).
+// It returns "" if no translation is found.
+// The algorithm for expanding short user names is as follows:
+// Look at the git commit log for the current repository,
+// extracting all the email addresses in Reviewed-By lines
+// and sorting by how many times each address appears.
+// For each short user name, walk the list, most common
+// address first, and use the first address found that has
+// the short user name on the left side of the @.
+func mailLookup(short string) string {
+	loadReviewers()
+
+	short += "@"
+	for _, r := range reviewers {
+		if strings.HasPrefix(r.addr, short) {
+			return r.addr
+		}
+	}
+	return ""
+}
+
+// loadReviewers reads the reviewer list from the current git repo
+// and leaves it in the global variable reviewers.
+// See the comment on mailLookup for a description of how the
+// list is generated and used.
+func loadReviewers() {
+	if reviewers != nil {
+		return
+	}
+	countByAddr := map[string]int{}
+	for _, line := range getLines("git", "log", "--format=format:%B") {
+		if strings.HasPrefix(line, "Reviewed-by:") {
+			f := strings.Fields(line)
+			addr := f[len(f)-1]
+			if strings.HasPrefix(addr, "<") && strings.Contains(addr, "@") && strings.HasSuffix(addr, ">") {
+				countByAddr[addr[1:len(addr)-1]]++
+			}
+		}
+	}
+
+	reviewers = []reviewer{}
+	for addr, count := range countByAddr {
+		reviewers = append(reviewers, reviewer{addr, count})
+	}
+	sort.Sort(reviewersByCount(reviewers))
+}
+
+type reviewersByCount []reviewer
+
+func (x reviewersByCount) Len() int      { return len(x) }
+func (x reviewersByCount) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
+func (x reviewersByCount) Less(i, j int) bool {
+	if x[i].count != x[j].count {
+		return x[i].count > x[j].count
+	}
+	return x[i].addr < x[j].addr
+}
+
 // stringList is a flag.Value that is like flag.String, but if repeated
 // keeps appending to the old value, inserting commas as separators.
 // This allows people to write -r rsc,adg (like the old hg command)
diff --git a/git-codereview/mail_test.go b/git-codereview/mail_test.go
index e3543df..f2baafe 100644
--- a/git-codereview/mail_test.go
+++ b/git-codereview/mail_test.go
@@ -4,7 +4,10 @@
 
 package main
 
-import "testing"
+import (
+	"fmt"
+	"testing"
+)
 
 func TestMail(t *testing.T) {
 	gt := newGitTest(t)
@@ -48,3 +51,57 @@
 
 	testMain(t, "mail", "-diff")
 }
+
+var reviewerLog = []string{
+	"Fake 1 <r1@fake.com>",
+	"Fake 1 <r1@fake.com>",
+	"Fake 1 <r1@fake.com>",
+	"Reviewer 1 <r1@golang.org>",
+	"Reviewer 1 <r1@golang.org>",
+	"Reviewer 1 <r1@golang.org>",
+	"Reviewer 1 <r1@golang.org>",
+	"Reviewer 1 <r1@golang.org>",
+	"Other <other@golang.org>",
+	"<anon@golang.org>",
+}
+
+func TestMailShort(t *testing.T) {
+	gt := newGitTest(t)
+	defer gt.done()
+
+	// fake auth information to avoid Gerrit error
+	auth.host = "gerrit.fake"
+	auth.user = "not-a-user"
+	defer func() {
+		auth.host = ""
+		auth.user = ""
+	}()
+
+	// Seed commit history with reviewers.
+	for i, addr := range reviewerLog {
+		write(t, gt.server+"/file", fmt.Sprintf("v%d", i))
+		trun(t, gt.server, "git", "commit", "-a", "-m", "msg\n\nReviewed-by: "+addr+"\n")
+	}
+	trun(t, gt.client, "git", "pull")
+
+	// Do some work.
+	gt.work(t)
+
+	testMain(t, "mail")
+	testRan(t,
+		"git push -q origin HEAD:refs/for/master",
+		"git tag -f work.mailed")
+
+	testMain(t, "mail", "-r", "r1")
+	testRan(t,
+		"git push -q origin HEAD:refs/for/master%r=r1@golang.org",
+		"git tag -f work.mailed")
+
+	testMain(t, "mail", "-r", "other,anon", "-cc", "r1,full@email.com")
+	testRan(t,
+		"git push -q origin HEAD:refs/for/master%r=other@golang.org,r=anon@golang.org,cc=r1@golang.org,cc=full@email.com",
+		"git tag -f work.mailed")
+
+	testMainDied(t, "mail", "-r", "other", "-r", "anon,r1,missing")
+	testPrintedStderr(t, "unknown reviewer: missing")
+}
diff --git a/git-codereview/review.go b/git-codereview/review.go
index cc8b937..6d1f6ac 100644
--- a/git-codereview/review.go
+++ b/git-codereview/review.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// TODO(adg): translate email addresses without @ by looking up somewhere
-
 // Command git-codereview provides a simple command-line user interface for
 // working with git repositories and the Gerrit code review system.
 // See "git-codereview help" for details.
@@ -237,6 +235,10 @@
 
 func dief(format string, args ...interface{}) {
 	printf(format, args...)
+	die()
+}
+
+func die() {
 	if dieTrap != nil {
 		dieTrap()
 	}
diff --git a/git-codereview/util_test.go b/git-codereview/util_test.go
index d9933ad..bc44e2f 100644
--- a/git-codereview/util_test.go
+++ b/git-codereview/util_test.go
@@ -147,6 +147,12 @@
 	}
 }
 
+func remove(t *testing.T, file string) {
+	if err := os.RemoveAll(file); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func trun(t *testing.T, dir string, cmdline ...string) string {
 	cmd := exec.Command(cmdline[0], cmdline[1:]...)
 	cmd.Dir = dir