Mail flow can be stopped for various reasons in a organization. Also it depends entirely on the environment design as there are various factors involved in affecting the mail flow like network, ports , firewall , antivirus , anti-spam , transport agents , directory services , connectors misconfiguration , exchange server services not running up and the list goes on.

Its always better to design the mail flow architecture  in a easy understandable way and also we need to ensure that the SMTP security inbound\outbound is tightened in the perimeter level to make sure no spam emails are circulated.

In this article i have mentioned few basic troubleshooting steps that can be followed during mail flow issues in a environment

This applies for both inbound/outbound mail flow issues

Following things can be done

1) First run EXBPA to check if we get any misconfig errors ( applies only for exchange 2007/2010). You can skip this step if you are running Exchange 2013 and upcoming versions.

2) Go through your event logs on hub transport if its 2010 , Mailbox Server if its 2013 to see if we get any clue (at times it may be a back pressure as well so its better to check logs). Its better to check all the exchange services at this time  and ensure if they are running.

3) Do a telnet from internal to external network and see if everything is fine and also perform telnet test from external domain to your domain.This test will usually help you to identify if there is any SMTP traffic block in your firewall.

Below is the example of performing a telnet test

Type Telnet domainname orIP 25

Above is an example of successful delivery to the target domain.

4)  Check whether the MX record is valid for the affected domain.

Below is an example of performing mx validation for google.com domain.

5) Enable protocol logging both send and receive connectors and see if you are able to track anything.

6) Check if  the connecting IP is in  blacklist

We need to obtain the following tool to do the check: