Let me explain. One of my customers experienced a loss of emails coming into their network yesterday. I started with the normal checking, making sure services were running, drive space, etc. The error that people emailing them were getting was an NDR saying Unknown User.
After a few hours of troubleshooting and removing email filters and such, my boss and senior engineer found a problem with the mx record in dns. It seems that the local ISP that hosts the web site for this company had a system wide problem yesterday with changes made to the mx records of all their customers' domains. This meant that everyone that was sending mail to this domain wasn't even getting to the server.
Seems they forgot to let us know about this outage.
So next time a hard problem comes up that you can't figure out, be sure to check all your basics because it may be staring you in the face like it was for me the whole time.
