Sometimes weird stuff happens with technology. There's no philosophical inquiry needed because, in my experience, technology problems often don't have an answer as to *why* the problem happened; it just happened. Yeah, there are some reasons we can give to why it happened, sometimes, but like with this problem I'm about to describe, I can't give a reason why it happened. All I know is that it happened and I had to fix it.
A couple of the employees told me they weren't receiving emails from our employee self service application. Employees use this to view paystubs, events, and such so it is important that this application is working correctly. Employees register with whatever email address they want to use: professional or personal, we don't care.
When I dove into Exchange 2010, I noticed by looking at the logs and the queue viewer that the only domain generating an error was our professional domain. Google, Live, and yahoo domains were receiving email from our server, but not our domain. The error generated in the queue viewer was 4126.96.36.199 DNS Query Failed. This was odd because I leave this server alone. The only changes made to the server are security updates. That's it. Regardless, this problem happened and I had to fix it.
How did I fix it? Well, obviously it was a DNS issue. I could tell by the error message. :p
So I checked out the external DNS lookups tab in the HUB transport server object properties. I had the top option "use network card dns settings" selected and that is how it has been running for over a year now. Anyway, since a problem had occurred I decided to select the other option "use these DNS servers" and put in the IPs of the servers we forward to. I then configured the SMTP send connector properties under the network tab (located by selecting hub transport under org. configuration) and checked the box "use the external DNS lookup settings on the transport server." Doing this cleared the queue and employees using their professional email addresses received (are are receiving) their email from "employee self service." Yeah, this is working, but I'm not sure why the problem occurred. I'm glad I was able to fix it in a timely manner. Obviously there was a problem with the DNS properties of the network card for the exchange server because changing the option to external lookups and using the DNS forwarders fixed the problem. I'll have to investigate further and will write here again if and when I figure that out.
Anyway, I hope this helps one of you having this strange problem.