THANK YOU FOR YOUR RESPONSE!
Turns out the error was made via a typo made on the end of the [company deliberately not named, as I'm not a fan of "smear campaigns"] when I upgraded my email service to the 1000 msgs-per-hour option.
Of course, since I'm such a lucky guy in terms of anything technologically-related (sarcasm deeply intact: I can't even get my 'avatar' to show up, for F's sake!), I had a glitch where over 1000 addys "disappeared" from my mailing list.
At least this time I have a back-up...it's the process of adding them all again that's a pain in my posterior I'd rather skip, please.&.thank.you!
Starting a new Topic re: problems with my WordPress library, but again: your help is greatly appreciated.