I am a very long-time user of SeaMonkey email, and have had a number of email providers, so I shouldn't be seeing noob-level troubles... or so I would have thought.
](./images/smilies/eusa_wall.gif "Brick wall"){kind=small}
EDIT: nothing to see here, move along.
Slightly longer version, in case it helps someone later: perhaps I have been using this too long, in any case, I fell prey to the "Set Default" button on the Outgoing Server (SMTP) page... of course this doesn't actually cause what one might think would happen to happen - you still need to explicitly select the new SMTP server entry in the top-level "Email Account" settings - or make sure the Account-level setting selects the "Default" SMTP server. Sigh.