Bad recipient address syntax when sending to an email ist | Thunderbird Support Forum | Mozilla Support
501 5.1.3 Bad recipient address syntax サーバーエラーでメールが送信できない場合の対処法 | PC上手
With email in outbox: Failed to send some messages. Negative SMTP reply 501 5.1.3 Bad recipient address syntax · Issue #1328 · thundernest/k-9 · GitHub