IEM配置退信设置无法连接iRedMail服务器的问题排查求助（含额外发件人验证错误）

各位大佬好，我遇到了一个关于Interspire Email Marketer（简称IEM）配置退信邮箱的问题，想请大家帮忙排查下：

环境说明 #

• IEM运行在Ubuntu 22.04版本的VPS上，配套PHP版本为7.3
• 目标退信服务器是部署了iRedMail、Postfix、Dovecot的独立VPS（IP标记为YYY）

核心问题现象 #

我把IEM里的退信邮箱设置成之前在另一台VPS上正常使用的同个邮箱，填写了正确的SMTP用户名、密码等参数后，点击「TEST BOUNCE SETTINGS」按钮，弹出如下错误提示：

Connection Timed Out
Verify your server is not firewalled
Try using a different protocol or port combination

已做的排查操作 #

• 临时关闭了两台服务器的防火墙进行测试，问题依旧
• 尝试了IMAP和POP3两种协议（官方推荐使用POP3），所有端口组合都试过，没有效果

退信服务器的日志信息 #

点击测试按钮时，退信服务器的mail.log里出现了如下记录（XXX是IEM服务器的源IP，YYY是退信服务器IP，已移除日期、时间和会话ID）：

mail dovecot: imap-login: Disconnected: Connection closed (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, session=
mail dovecot: imap-login: Disconnected: Connection closed (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, TLS handshaking: Connection closed, session=
mail dovecot: imap-login: Disconnected: Connection closed (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, TLS handshaking: Connection closed, session=
mail dovecot: imap-login: Disconnected: Connection closed (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, TLS handshaking: Connection closed, session=
mail dovecot: imap-login: Disconnected: Connection closed (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, TLS handshaking: Connection closed, session=
mail dovecot: imap-login: Disconnected: Aborted login by logging out (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, session=
mail dovecot: imap-login: Disconnected: Aborted login by logging out (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, session=
mail dovecot: imap-login: Disconnected: Too many invalid commands (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, session=
mail dovecot: imap-login: Disconnected: Too many invalid commands (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, session=
mail dovecot: imap-login: Disconnected: Too many invalid commands (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, session=
mail dovecot: imap-login: Disconnected: Too many invalid commands (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, session=
mail dovecot: imap-login: Disconnected: Connection closed (no auth attempts in 0 secs): user=<>, rip=XXX, lip=YYY, TLS handshaking: Connection closed, session=

补充信息 #

如果我改用共享主机上的另一个退信邮箱，测试是正常的，也能正常处理退信，但我还是希望能使用YYY这台服务器作为退信服务器。目前这台退信服务器的其他邮件功能都正常，唯独IEM无法连接到它的退信设置。另外我已经在退信服务器的Fail2Ban里把IEM的IP（XXX）加入白名单，不会被封禁。

附加问题 #

还有个额外的小问题：当我使用共享主机的退信邮箱时，虽然退信能正常处理，但用户通过IEM注册时会报错：Recipient address rejected: Sender is not same as SMTP authenticate username。我尝试过针对这个错误的两种常规解决方案，但都没能解决问题。

麻烦各位大佬帮忙看看可能的原因，谢谢！

备注：内容来源于stack exchange，提问作者Hawk007