Fail to send mails
1. Reasons why system mail returns
During mail transmission process, if you receive system mail return, please check the content and compare to the next few situations, find out the reason. Generally, here are some common reasons for system mail return:
1) Addressee’s address is not found
Invalid address (#5.5.0)/User unknown/user is not found/doesn't have this account
Error cause: the address doesn’t exist or the mail address input is incorrect.
Solution: check the address and send again.
2) Recipient mailbox has no enough space to receive new mails
Mailbox is full/Quota exceed the hard limit for user/mail box space not enough
Error cause: recipient mailbox is full, there’s no enough space to receive new mails.
Solution: clear up and preserve enough space to receive to receive the mail.
3) Exceed the mail size limitation fixed by the mail service supplier
552 Message size exceeds fixed limit
Error cause: the mail is too big, exceeding the limitation for a mail specified by mail service supplier.
Solution: divide the attachment into several parts and send, or let the recipient change a bigger mailbox to receive the mail.
4) Rejected by the server
Recipient address rejected/552 Error: content rejected/553 Mail data refused
Error cause: rejected by mail server
Solution: inform the user of the concrete reason, and let the recipient network manager to explain the situation.
5)  Server could not build connection
Sorry, I wasn't able to establish an SMTP