Add SMTP as common initialism.

Fixes #49.

Signed-off-by: David Symonds <dsymonds@golang.org>
diff --git a/lint.go b/lint.go
index d85cd6b..b8cb605 100644
--- a/lint.go
+++ b/lint.go
@@ -498,6 +498,7 @@
 	"RHS":   true,
 	"RPC":   true,
 	"SLA":   true,
+	"SMTP":  true,
 	"SSH":   true,
 	"TLS":   true,
 	"TTL":   true,