fix parsing message headers with addresses that need double quotes

we are using Go's net/mail to parse message headers. it can parse addresses,
and properly decodes email addresses with double quotes (e.g. " "@example.com).
however, it gives us an address without the double quotes in the localpart,
effectively an invalid address. we now have a workaround to parse such
not-quite-addresses.

for issue #199 reported by gene-hightower, thanks for reporting!

message/part.go

@@ -497,9 +497,9 @@ func parseAddressList(log mlog.Log, h mail.Header, k string) []Address {
 	for _, a := range l {
 		// todo: parse more fully according to ../rfc/5322:959
 		var user, host string
-		addr, err := smtp.ParseAddress(a.Address)
+		addr, err := smtp.ParseNetMailAddress(a.Address)
 		if err != nil {
-			log.Infox("parsing address (continuing)", err, slog.Any("address", a.Address))
+			log.Infox("parsing address (continuing)", err, slog.Any("netmailaddress", a.Address))
 		} else {
 			user = addr.Localpart.String()
 			host = addr.Domain.ASCII

message/part_test.go

@@ -596,3 +596,10 @@ func TestEmbedded2(t *testing.T) {
 	_, err = EnsurePart(pkglog.Logger, false, bytes.NewReader(buf), int64(len(buf)))
 	tfail(t, err, nil)
 }
+
+func TestNetMailAddress(t *testing.T) {
+	const s = "From: \" \"@example.com\r\n\r\nbody\r\n"
+	p, err := EnsurePart(pkglog.Logger, false, strings.NewReader(s), int64(len(s)))
+	tcheck(t, err, "parse")
+	tcompare(t, p.Envelope.From, []Address{{"", `" "`, "example.com"}})
+}

sendmail.go

@@ -201,7 +201,11 @@ binary should be setgid that group:
 						if len(addrs) != 1 {
 							log.Fatalf("only single address allowed in To header")
 						}
-						recipient = addrs[0].Address
+						addr, err := smtp.ParseNetMailAddress(addrs[0].Address)
+						if err != nil {
+							log.Fatalf("parsing address: %v", err)
+						}
+						recipient = addr.Pack(false)
 					}
 				}
 				if k == "to" {

smtp/address.go

@@ -176,6 +176,21 @@ func ParseAddress(s string) (address Address, err error) {
 	return Address{lp, d}, err
 }
 
+// ParseNetMailAddress parses a not-quite-valid address as found in
+// net/mail.Address.Address.
+//
+// net/mail does parse quoted addresses properly, but stores the localpart
+// unquoted. So an address `" "@example.com` would be stored as ` @example.com`,
+// which we would fail to parse without special attention.
+func ParseNetMailAddress(a string) (address Address, err error) {
+	i := strings.LastIndex(a, "@")
+	if i < 0 {
+		return Address{}, fmt.Errorf("%w: missing @", ErrBadAddress)
+	}
+	addrStr := Localpart(a[:i]).String() + "@" + a[i+1:]
+	return ParseAddress(addrStr)
+}
+
 var ErrBadLocalpart = errors.New("invalid localpart")
 
 // ParseLocalpart parses the local part.