When you’re troubleshooting mail flow, before you look elsewhere, you should at some point check to see if your server is actually delivering mail or not. There is no better way to do so than using a Telnet command.
Those of us who work in the IT field know about this command; others may have just heard about it and even then might not know what it is used for.
Telnet is a very basic command line and very easy to use. Below is an example of a complete message that is sent through Telnet.
PLEASE NOTE: if you try this make sure NOT to make any TYPOS as Telnet does not forgive backspaces or any type of deletion after you typed it. If you make any typos you must start again from scratch.
So, from your server or computer that runs an SMTP service:
1- Open the command prompt (Start>run and type: cmd and hit <Enter>)
2- Type : telnet Server_IP 25 <Enter>
3- Notice that you get a banner that displays a positive response of 220 and the version of the SMTP you are using or your customized banner.
4- From this line anything you see in BLUE is what you need to type, in BLACK is what the system responds:
5- After the Quit command the system generates a message_ID, then Quit the telnet command by bringing you back to the C:> prompt, then delivers the message to the destination mailbox. At this point you should get the message, if not then your mail server might be the culprit.