Email SMTP Error Codes/Bouncebacks

If you or someone trying to send an email to you are getting a bounceback (and automated email saying the message could not be delivered) the reason will be included in the bounceback in code form.
 
These codes can mean a number of different things so we've listed some of the more common so you can diagnose any issues. Depending on the servers involved, the bounceback message might be worded slightly differently. 
 

450 4.7.1 Recipient address rejected: Access denied
This occurs when the recipient address is invalid. i.e., not in form of user@domainname.com OR if the Recipient address is blocked on server.

522 5.7.1 : Recipient address rejected: Requested mail action aborted: exceeded storage allocation
This bounceback indicates the recipient mailbox is over its quota for storage and the message is being rejected as a result.

530 5.7.0 Recipient address rejected: Authentication Required

This bounceback indicates that the originating sender is sending mails to remote domain without using smtp authentication.

550 5.1.1 Recipient address rejected: User unknown in virtual alias table
 
This is the bounceback received if the user being sent the email is not a known mailbox on that domain. It can be down to a typo in the senders 'to' field, but could also be down to a number of other factors. 
 
553 5.7.1 sorry, that domain isn't in my list of allowed rcpthosts

This will be probably due to an incorrect setting on your outgoing server settings. Ensure you have 'My Outgoing server (SMTP) requires authentication ticked, and the 'Use same settings as my incoming mail server' (or similar) selected.

554 5.7.1 Relay access denied

This will be probably due to an incorrect setting on your outgoing server settings. Ensure you have 'My Outgoing server (SMTP) requires authentication ticked, and the 'Use same settings as my incoming mail server' (or similar) selected.
 
The server responded: rblsmtpd: (IP ADDRESS) 451 Spamhaus
 
This bounceback indicates that the sender or recipient has been blacklisted for spam. You can check the domain for any blacklistings at http://mxtoolbox.com. The owner of the domain will need to investigate why the IP Address has been blacklisted - it can sometimes be because a website or mailbox on that server has been compromised and will need fixing before the provider will delist. 
 
If you are having an email issues on your server with us, please drop our support team a line on 0208 0909664.

Note: Your server with Nimbus is capable of being used as a basic mail server, but the ongoing battle between spam management, hacking and blacklisting means that Office 365 and Google Apps are the go-to platforms if email is critical to your business/site. Read more here.
 
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