Sometimes mail flow from your email server can stop
, or at least appear to stop, altogether for no apparent reason and finding out why your mail float has stopped can prove to be a bit tricky.
Delivering mail is not as straight forward a job as you might think, even if the message itself does arrive at its destination server it may still be rejected for purely technical reasons and so the following checks should be made as a matter of routine when mail stops arriving.
Check that your MX records (Mail Exchanger Records) for your domain are configured correctly.
Check that your server is not listed on Spamhaus, SORB's or Spamcop blacklists
Check that your PTR record is set correctly
Check that you can actually connect to your mail server via port 25 (the port used by SMTP)
Use online tools, such as Check SMTP, to discover any problems at the SMTP level and the recipient mailbox is ready to process mail.
If all the above checks are found to be working your next step is to move onto the Message Transfer Agent (MTA) or your Anti-Spam software.
For the most part SMTP can report any faults the MTA may encounter but it is not always guaranteed, especially when using 3rd party mail filtering and smart hosts. If you are using some gateway servers for filtering mail or as a smart host you should check the SMTP services behind your front line servers to be sure they can also accept mail for your domains.
If all SMTP services are found to be working your next step is to move onto any Anti-Spam software you may have installed. Often Anti-Spam solutions can silently drop messages when they arrive at the server or at the end users mail client. The simplest test would be to disable the anti-spam for a short period while you send an email from an address which appears to be blocked. If the message arrives then something within the Anti-Spam solution is configured inappropriately or there is a problem with the message, sender or senders mail server.
If your message still fails to arrive and you have not received a Non Delivery Report (NDR) you are left with the Message Transfer Agent.
Each mail server has its own tools for tracking messages as they go through the MTA and you should refer to its help documentation.