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.
Best Answer
The message you posted is a failure to connect which doesn't necessarily mean they refused your email, it seems more likely a dns issue or some other connection issue. Some things to check/think about are:
Can you check your application server to make sure it is properly resolving the dns and MX records for the domain you are trying to email.
Can you telnet from this server to those domains, if so what response do you get?
Do the domain names you are sending from have spf records and if so, is the ip you are sending from included in the spf record?
Are reverse DNS records setup properly for the IP you are sending from for the domains you are sending as?
Ideally, before anyone can really give you a definitive answer on the problem you need to have a telnet or something and get a server response telling you why you cannot connect in the first place.