Symptoms: when sending a mail to email@example.com, you may receive undeliverable
message like this: <mail.chicagotech.net> #5.7.1 smtp;550 5.7.1
<firstname.lastname@example.org>... Relaying denied. IP name lookup failed (x.x.x.x).
1. DNS lookup failed
2. The sender of the message does not have the privileges required to complete delivery.
3. You are trying to relay your mail through another SMTP server and it does not permit you to relay.
4. The recipient might have mailbox delivery restrictions enabled.
5. Make sure you mail server is no on a spam list.
6. If there are a lot errors and warning such as ID 104, 215 related to spam and virus in the Application Event Viewer
of the Exchange server, you may need to restart the Exchange services.