看退信下面的日志，并按照相应的错误信息进行匹配。

511 sorry, no mailbox here by that name

unknown user account

... User unknown

Name server reports domain name unknown

mailbox unavailable

550 Invalid User:

invalid recipient

No such user here

All recipients are invalid

User unknown in virtual alias table

Mailbox not found

Recipient address rejected: User unknown in relay recipient table

以上信息均为收件人地址错误或者无效，检查收件人地址，检查邮件服务器的域名解析服务器

554 Connection refused(mx). MAIL FROM [test@test.com] mismatches client IP [74.86.11.12].

域名和IP 地址不对应导致的拒收 [上面的域名和IP 地址会变化，类似的情况]，修正您的域名解析，域名解析请参考：http://www.comingchina.com/windows/install/954.html

553 5.7.1 SpamTrap=reject mode, dsn=5.7.1, Message blocked by BOX Solutions (www.box-sol.com)

SpamTrap Technology, please contact the MUSTEK site manager for help: (bkndr63272).

如果出现类似上述的信息是因为您发了不受对方欢迎的信件，导致对方拒收

551 User not local; please try

邮件被GFW 过滤，GFW 是指Great Firewall，中国国家防火墙。尝试将邮件内容打包压缩或者放入OFFICE 文档内进行邮件传输

Winsock Error 10053 Connection abort.

网络通讯故障

Winsock Error 10060 The connection timed out.

连接超时，网络通讯故障造成

Winsock Error 10061 The connection timed out.

对方服务器拒绝连接，对方服务器主动断开链接，这通常是由国外不活跃的服务器造成的。

452 4.3.1 Out of memory

This message is 60 minutes old; it has 0 minutes left in this queue

Remote queue lifetime exceeded; message placed in retry queue

一般情况下，邮件会进入重试队列进行再次发送，这种情况下对方的邮件服务器一般是Exchange，这是Exchange 的内部错误造成的。

553 Requested action not taken: no smtp MX

收件人邮箱地址错误，域名无法被解析,检查收件人地址，或者对方域名解析设置错误。

501 syntax.helo hostname

501 Invalid domain name

502 unimplemented command

503 5.0.0 polite people say HELO first

533 relay restriction

544 <>:Recipient address rejected: Relay access denied

传输中的语法错误，原因不明。