I think it's fair that we reset the Msg.senderror to nil before we Client.Send it.

Since the send error indicate an error during the mail delivery, in my opinion it should be reset when a re-send is initiated, so that the senderror field always represents the latest delivery error. The send error should be checked after mail delivery and before a retry is started.

client_119.go

@@ -18,6 +18,7 @@ func (c *Client) Send(ml ...*Msg) error {
 		return fmt.Errorf("failed to send mail: %w", err)
 	}
 	for _, m := range ml {
+		m.sendError = nil
 		if m.encoding == NoEncoding {
 			if ok, _ := c.sc.Extension("8BITMIME"); !ok {
 				errs = append(errs, ErrServerNoUnencoded)

client_120.go

@@ -19,6 +19,7 @@ func (c *Client) Send(ml ...*Msg) (rerr error) {
 		return
 	}
 	for _, m := range ml {
+		m.sendError = nil
 		if m.encoding == NoEncoding {
 			if ok, _ := c.sc.Extension("8BITMIME"); !ok {
 				rerr = errors.Join(rerr, ErrServerNoUnencoded)