courier: Use the senders' domain when saying EHLO

Some servers, like postfix, will pay close attention to the domain we say as a
part of the EHLO.

By default, Go's smtp package will use "localhost", causing it to complain.

This patch fixes that by using the envelope-from's domain.
It's not clear if that's better than using what we are resolving to, but
that's much more involved so we're going to do this for now.

internal/courier/smtp.go

@@ -67,6 +67,12 @@ retry:
 		return tr.Errorf("Error creating client: %v", err), false
 	}
 
+	// Issue an EHLO with a valid domain; otherwise, some servers like postfix
+	// will complain.
+	if err = c.Hello(envelope.DomainOf(from)); err != nil {
+		return tr.Errorf("Error saying hello: %v", err), false
+	}
+
 	// TODO: Keep track of hosts and MXs that we've successfully done TLS
 	// against, and enforce it.
 	if ok, _ := c.Extension("STARTTLS"); ok {