. A customer calls in with an e-mail issue. Mail flow is working internally, and has been validated already technically. The user is having trouble sending an e-mail to a particular user on a large e-mail hosting service like Yahoo or G-Mail. The SMTP bounce back error is an undeliverable 5.3.0. It looks like the mail server may be blacklisted.
What are some troubleshooting steps you need to take to diagnose and resolve this issue?