allow empty envelope (#166)

* feat: allow empty MAIL FROM

Closes #164

pkg/server/smtp/handler.go

@@ -41,7 +41,7 @@ const (
 // accepting '>' as quoted pair and in double quoted strings (?i) makes the regex case insensitive,
 // (?:) is non-grouping sub-match
 var fromRegex = regexp.MustCompile(
-	"(?i)^FROM:\\s*<((?:\\\\>|[^>])+|\"[^\"]+\"@[^>]+)>( [\\w= ]+)?$")
+	"(?i)^FROM:\\s*<((?:(?:\\\\>|[^>])+|\"[^\"]+\"@[^>])+)?>( [\\w= ]+)?$")
 
 func (s State) String() string {
 	switch s {
@@ -314,11 +314,14 @@ func (s *Session) readyHandler(cmd string, arg string) {
 			return
 		}
 		from := m[1]
-		if _, _, err := policy.ParseEmailAddress(from); err != nil {
+		if _, _, err := policy.ParseEmailAddress(from); from != "" && err != nil {
 			s.send("501 Bad sender address syntax")
 			s.logger.Warn().Msgf("Bad address as MAIL arg: %q, %s", from, err)
 			return
 		}
+		if from == "" {
+			from = "unspecified"
+		}
 		// This is where the client may put BODY=8BITMIME, but we already
 		// read the DATA as bytes, so it does not effect our processing.
 		if m[2] != "" {

pkg/server/smtp/handler_test.go

@@ -79,6 +79,35 @@ func TestGreetState(t *testing.T) {
 	}
 }
 
+// Test commands in READY state
+func TestEmptyEnvelope(t *testing.T) {
+	ds := test.NewStore()
+	server, logbuf, teardown := setupSMTPServer(ds)
+	defer teardown()
+
+	// Test out some empty envelope without blanks
+	script := []scriptStep{
+		{"HELO localhost", 250},
+		{"MAIL FROM:<>", 250},
+	}
+	if err := playSession(t, server, script); err != nil {
+		// Dump buffered log data if there was a failure
+		_, _ = io.Copy(os.Stderr, logbuf)
+		t.Error(err)
+	}
+
+	// Test out some empty envelope with blanks
+	script = []scriptStep{
+		{"HELO localhost", 250},
+		{"MAIL FROM: <>", 250},
+	}
+	if err := playSession(t, server, script); err != nil {
+		// Dump buffered log data if there was a failure
+		_, _ = io.Copy(os.Stderr, logbuf)
+		t.Error(err)
+	}
+}
+
 // Test commands in READY state
 func TestReadyState(t *testing.T) {
 	ds := test.NewStore()