错误原因：因为收件人邮箱帐号不存在或者收件方邮件服务器无法找到收件人邮件地址，所以对方服务器不接收此封邮件。
可能出现的关键字：No such user、user unknown、unknown or illegal alias、account inactive、user not found、Invalid address、invalid recipient、not a valid mailbox、mailbox cannot be delivered、invalid rcpt to等。
可能出现的退信实例：
1) 550 Mailbox not found. http://service.mail.qq.com/cgi-bin/help?subtype=1&&id=20022&&no=1000728
2) 550 5.1.1 xxx@abcd.com: Recipient address rejected: User unknown
3) 550 5.1.1 Mailbox does not exist
4) 550 Mailbox not found
5) 550 User not found: 12345@163.com
6) 550 RC:LD The email account that you tried to reach does no exists.
7) 551 5.1.1 recipient is not exist
8) 501 Bad address syntax. http://service.mail.qq.com/cgi-bin/help?subtype=1&&id=20022&&no=1000730
9) 550 #5.1.0 Address rejected.
10) 550 5.1.1 1111@ss.com: Recipient address rejected: undeliverable address: unknown user: "1111@ss.com"
11) 550 abcd:user not exist
12) 550 5.1.1 Bad destination mailbox address
13) 550 Local User [xxx@123.com] not found.
14) 550 no such user: ****@***.dbmailtest.**
15) 550 #2175041 unavailable mail user
16) 550 #5.1.0 Address rejected.
17) 552 "RCPT TO" mailbox unavailable
18) 550 5.1.1 ***@dbmailtest recipient rejected
19) 550 5.7.0 ***@dbmailtest... address invalid
20) 550 RC:NE The email account does not exist.
21) 550 no such user here (umail mta)
22) 550 5.1.1 xxx@abcd.com: Recipient address rejected: User unknown in virtual mailbox table
23) 550-5.1.1 The email account that you tried to reach does not exist. Please try
24) 550-5.1.1 double-checking the recipient's email address for typos or
25) 550-5.1.1 unnecessary spaces. Learn more at
26) 550 5.1.1 http://support.google.com/mail/bin/answer.py?answer=6596 vw5si4655694pab.333 - gsmtp
27) 550 5.2.1 The email account that you tried to reach is disabled.
28) 550 Requested action not taken: mailbox unavailable
29) 5.1.0 - Unknown address error 550-'5.7.0 user access deny.'
30) ***@yahoo.com, 554 delivery error: dd This user doesn't have a yahoo.com account (***@yahoo.com) [-5] - mta155.mail.sc5.yahoo.com
31) 550 Invalid recipient: <***@index.com.jo> (RCPT TO:***@index.com.jo)
32) 550 ***@szonline.net unknown user account (RCPT TO:***@szonline.net)
33) 550 5.1.1 <***@melco.com.hk>... User unknown (RCPT TO:***@melco.com.hk)
34) …… invalid address
35) : host ……said: 550 5.7.1 Unable to relay for dbmailtest@126.com:host……said: 550 5.1.1 is not a valid mailbox
36) : host smtp.china.com[61.135.144.88] said: 550 , No such user.
37) ……user account inactive
38) …… unknown or illegal alias:dbmailtest
39) : User ***@163.net is not found in Server.
40) : host ……said: 550 Invalid recipient
解决方法：
1) 请确认收件人地址是否正确
2) 联系收件人，确认邮箱地址是否存在。
确认无误后重新发送邮件即可。
