One of my users was suddenly unable to deliver emails and was getting the error 554 accompanied by the text "Sorry your envelope sender is in my bad mail list". After digging deeper, I found that test messages are failing as well with the error "unable to send test message. Verify the e-mail address field." When I ran the test by replacing his e-mail address with one of my own, the test runs successfully, and the test message gets delivered to my mailbox!
I have attempted installing his account on another computer(behind different router), and have gotten it to send successfully. Furthermore, he has also been successful sending emails from a browser. Other people in the same organization behind the same router have no problems sending an email. The person who runs the email servers thinks that my user's address is on some e-mail blacklist. However, I've check the blacklists using mx-tool, and his domain is clean. Furthermore, I don't think blacklists work on a per user level. I've tried rebuilding his profile and resetting Outlook with no success.