Mail delivery failed: returning message to sender

If your domain name is hosted by a 3rd party on a shared server, you may already have received this very frustrating email seconds after you pushed the send button:
Mail delivery failed: returning message to sender

The typical situation is the following: your hosting provider hosts several customers on the same shared Linux or Windows server and one of these accounts gets infected by a trojan or bot that spams the rest of the world. Since you are sharing the same IP address, when the IP address becomes blacklisted on spamhaus or cbl, whenever you try to send a email, the receiving server rejects it and you will very likely receive the following message:

Subject: Mail delivery failed: returning message to sender
Header: This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
SMTP error from remote mail server after initial connection:
host yyy.zz [aaa.bbb.ccc.ddd]: ...
------ This is a copy of the message, including all the headers. ------
Received: from...

See Spamhaus for details.

What to do next?
The first thing that you need to do is to contact the technical support department of your hosting provider to inform them of the situation. It is very likely that they will ask for the full header of the message you just received back. Save yourself some time and copy/paste it in your ticket.

Then, you wait…

If you are frustrated by this situation, you can always visit the following services to check whether the IP address of your server is or not blacklisted:
1. CBL query page
2. query page

I’d recommend to not try to delist the IP address yourself. It’s there for good reasons and your hosting provider may need to see why the IP is blocked.

Usually the IP can be removed within an hour and the email servers sync their lists often (as often as twice an hour). If your provider is efficient you should have the situation resolved within 24h. In the meantime, gmail and yahoo mail are your friend…

Good luck!

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