After that I was able login to the webemail through https://www.mydomain.com:20000/ and send an email to another account which is located on a different server. Then when I replayed to that email I never received that email and other server after some time returns delivery delay message:
"This message has not yet been delivered. Microsoft Exchange will continue to try delivering the message on your behalf. "
I have checked mail logs but I see only local user activity (log in, check inbox, message sent) but nothing to deliver.
Domain name MX record is pointed to the server IP.
From the MXToolbox:
“SMTP Reverse DNS Resolution. Reverse DNS FAILED! This is a problem.”
The error message sounds like the IP address of your server has no PTR record. This is usually assumed to be a good sign of abuse (i.e. a compromised end user computer trying to deliver email) by mail servers. But in this case, it’d be the other way round: You wouldn’t be able to SEND email, instead of not receive it.
To debug this further, I’d need to know the exact domain name and IP address in question, and an email address at that domain where I can send a test mail to, to see what happens on the sending end, if you say that you see nothing in your mail logs when a delivery attempt should take place.