Email provider · 5.x.x permanent failure
Every SMTP 550 response on record from QQ, with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
The mailbox is unavailable and the message is rejected for good. This is the catch-all permanent failure, covering everything from an unknown address to an authentication failure to a reputation block, so the wording after the number is what actually tells you the reason.
smtp;550 Mail is rejected by recipients
The receiver refused this message outright and will not accept an unchanged retry. The wording above is the only clue to what needs to change, most often the address, authentication, or sending reputation.
Provider documentation
smtp;550 Mailbox unavailable or access denied
The address does not exist, or is no longer active, at this provider. Remove it from your list and check for a typo in the local part or the domain.
Provider documentation
smtp;550 Mailbox not found. http://service.exmail.qq.com/cgi-bin/help?subtype=1&&id=20022&&no=1000728
The receiver refused this message outright and will not accept an unchanged retry. The wording above is the only clue to what needs to change, most often the address, authentication, or sending reputation.
Provider documentation
smtp;550 Mail content denied. http://service.exmail.qq.com/cgi-bin/help?subtype=1&&id=20022&&no=1000726
The receiver refused this message outright and will not accept an unchanged retry. The wording above is the only clue to what needs to change, most often the address, authentication, or sending reputation.
Provider documentation
smtp;550 Sender frequency limited. http://service.exmail.qq.com/cgi-bin/help?subtype=1&&id=20022&&no=1000723
You are sending to this receiver faster than it will accept from your IP or domain, and it has stopped taking the message. Cut the send rate, spread volume over a longer window, and requeue.
Provider documentation
smtp;550 Suspected spam sender.
The receiver's spam filter scored the message above its rejection threshold. Review the content and links, build sending reputation gradually, and only mail people who asked to hear from you.
Provider documentation
smtp;550 DMARC check failed http://service.mail.qq.com/cgi-bin/help?subtype=1&&no=1001508&&id=16.
The message did not pass the sending domain's DMARC policy, so the receiver rejected it. Sign the mail with DKIM for your From domain, or align SPF through a custom Return-Path, then send again.
Provider documentation
Postmaster & documentation
The mailbox is unavailable and the message is rejected for good. This is the catch-all permanent failure, covering everything from an unknown address to an authentication failure to a reputation block, so the wording after the number is what actually tells you the reason. At QQ, it is a permanent rejection, and the specific reason is in the enhanced status code and message text of each response below.
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check QQ's postmaster documentation.
