I have just installed an smtp relay server that will only be used by apps behind our firewall to send email. I've called it smtprelay.mydomain.com and this is a valid dns entry that is pointing to my firewalls public ip.
It seems to work fine and I can send mail to gmail, hotmail and other domains.
My problem is that my ISP is blocking incoming port 25 traffic, so no one is able to connect from the net to smtprelay.mydomain.com on port 25. Could this be a problem? Do some servers connect back to verify that the sender server exists? Is there a risk that we could be marked as a source of spam?