If you have a mail server that you are having problems with, the easiest way to test it is using Telnet:
- Launch APPS AND FEATURES
- Install TELNET
- Windows 10 – PROGRAMS AND FEATURES > TURN WINDOWS FEATURES ON OR OFF
- Windows 2016 – SERVER MANAGER > LOCAL SERVER > MANAGE > ADD ROLES AND FEATURES > Telnet is a FEATURE
- Launch CMD
- always a good idea to RUN AS AN ADMIN, but not required
- Type
TELNET <MAIL.DOMAIN.COM> 25
and press the enter key- this is not case sensitive
- obviously, substitute your mail server in the <>
If it fails you will likely see Could not open connection to the host, on port 25: Connect failed
- If you have access to the mail server you can check the firewall to make sure port 25 is open
- You should then ping the mail server to see if your DNS has the correct IP entry for it
- You can also try to TRACERT to that mail server to see where it is failing
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