Dovecot与Postfix配置Submission服务时认证失败问题求助

各位大佬好，我最近在配置Dovecot+Postfix的Submission服务时碰到了个棘手的问题，想请大家帮忙排查下：

• 我用命令 doveadm auth login user pass 测试Dovecot的认证功能，能得到正确的输出，说明Dovecot本身的认证逻辑是正常的
• 但每当我尝试通过Submission（或Submissions）端口发送邮件时，都会收到认证失败的错误提示

以下是我的doveconf -n输出内容：

auth_mechanisms = plain login
disable_plaintext_auth = no
mail_location = maildir:/home/vmail/%d/%n
mail_privileged_group = vmail
namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix =
}
passdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
protocols = " imap lmtp submission"
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0666
    user = postfix
  }
}
service lmtp {
  unix_listener /var/spool/postfix/private/lmtp {
    group = postfix
    mode = 0600
    user = postfix
  }
}
ssl_cert = < some_crt
ssl_client_ca_dir = /etc/ssl/certs
ssl_dh = # hidden, use -P to show it
ssl_key = # hidden, use -P to show it
userdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}

目前我暂时没找到问题出在哪，有没有大佬能指点下方向？比如是不是Postfix这边的Submission相关配置没和Dovecot的auth服务正确对接上？

备注：内容来源于stack exchange，提问作者Andrei