Sandy, you are completely correct. Turns out the problem is my e-mail provider suddenly started filtering in an unusably aggressive manner, without warning. Thank you, and sorry for wasting your time.
In continuing to track down the problem, here's what we've found so far:
The IP address 23.27.50.10 (mailfwd1.zoneedit.com) is on the Backscatterer.orgblacklist. This is how the mails ended up getting blocked from the entire mail server (and thus my inbox).
Is this a configuration issue on your end perhaps, making the mailforwarder susceptible to abuse, or is it just the nature of a mailforwarder?
We do not believe they are an ethical RBL and do not have the best interests of the overall Internet community at heart. They charge a fee to be removed from their list and as such are in it for the money, not necessarily the protection of mail integrity. You might want to consider reporting the bogus result to the service provider that gave them this information.