It took a few days to figure it out, but I think I've finally fixed all of my e-mail problems. It appears that messages that I had sent were not being properly delivered. Unfortunately, I had no indication that there was a problem, and it may have been going on as long as two months. If you tried to reach me recently and received no reply, I apologize.
I should be able to receive and send mail now from my primary e-mail address, firstname.lastname@example.org.