I have this asp.net application that sends out emails. The SMTP server is iis6. The emails are being sent on behalf of our application users and they have different domain names. Most of the email providers, like gmail and yahoo, would accept the emails but some of them not e.g. AOL.
This is the message that users receive from our SMTP server after a few minutes:
Subject: Delivery Status Notification (Failure) This is an automatically generated Delivery Status Notification. Unable to deliver message to the following recipients, due to being unable to connect successfully to the destination mail server.