按照下列“Q”代表问题，“A”代表解答。
Q:the server says:550 relaying mail to <> is not allowed
Q:The server says:550 <>... relaying denied
Q:the server says:550 5.7.1 relaying not permitted:
A:使用某些Smtp服务器时，限制了收件人的地址，只能换一个Smtp服务器。
Q:The server says:550 <>:local user only
Q:The server says:550 <>:Invalid User
Q:The server says:550 Invalid recipient
A:使用163.net，163.com，yeah.net和netease.com之类的Smtp服务器时，只能用自身的信箱发信，所以要在Foxmail的“帐户属性”中的“个人信息”里面填写正确的邮件地址。
Q:the server says:551 delivery not allowed to non-local recipient
Q:The server says:553 Relay restriction. Q:The server says:553 From <>, message blocked.
Q:The server says:553 sorry,you are not allow to use this SMTP to relay your eami
Q:The server says:553 sorry, that domain isn't in my list of allowed rcpthosts
A:使用21cn.com，china.com，371.net，sina.com等大多数信箱的smtp服务器时，只能用自身的信箱发信，所以要在Foxmail的“帐户属性”中的“个人信息”里面填写正确的邮件地址。
Q:The server says:505 client was not authenticated
Q:The server says:553 authentication is required to send mail as <>
A:使用263.net和sohu.com的Smtp服务器时，不但要用自身的邮箱发信，而且要加入身份验证，所以即要在“个人信息”中填写正确邮箱地址，又要选中“S MTP服务器需要认证”。
Q:The server says:535 Error:authenticatin failed
Q:The server says:535 Authentication unsuccessful
Q:The server says:452 Insufficient system storage
A:在使用Esmtp认证的过程中出错，检查一下Esmtp设置，多试几次。
Q:The server says:553 <>...domain name required
Q:The server says:550 Unable to relay for ...
A:多出现在用Wingate代理服务器发送邮件时。虽然在Wingate中“POP3 邮箱帐号”要使用“用户名#POP3地址”的格式，但在“帐户属性”中的“个人信息”中还是要填写一般的格式。
Q:The server says:553 mailbox name not allowed
A:收件人邮箱地址不允许，需检查收件人地址是否正确。
Q:the server says:553 sorry, your envelop sender is in my badmailfrom list
A:服务器限制了收件人的地址，只能换一个smtp服务器发信。
Q:the server says:554 Transaction failed
Q:The server says:451 Requested action aborted,errno=28 A:传输失败，检查网络问题。
Q:the server says:503 error:needmail command Q:The server says:503 need mail before RCPT.
Q:The server says:503 Bad sequence of commands
A:消息命令顺序出错，一般出现在其他错误之后，先检查之前出现的错误提示。
Q:The Server says:501 syntax.helo hostname Q:The server says:501 Invalid domain name
Q:The server says:502 unimplemented command
Q:the server says:503 5.0.0 polite people say HELO first
Q:The server says:533 relay restriction
Q:The server says:544 <>:Recipient address rejected: Relay access denied
A:传输中的语法错误，原因不明。

参考：
https://zhidao.baidu.com/question/536780983.html