I am running sendmail.
It is running behind a firewall with the proper ports being forwarded (25,110) to the correct box. I am getting occasional failures receiving e-mail from the same address, ie paypal payment notices are getting through about 30%-50% of the time. Out of 5 e-mails sent from paypal, I may only get 2-3. Another example is my home address from sbcglobal.net is failing, but a coworkers from the same provider is getting through. I was thinking some DNS issue, but DNS appears to be working. Any thoughts?