Taz, I'm just wondering before I try your option 3—re-creating my e-mail from scratch—if there's a bit more to this. It strikes me that if I simply delete my email accounts and then add them back in, the offending bit of Mail that's causing the prob with smart mailboxes might still be lurking somwhere apart from the accounts themselves. So my question: Is there anything additional to do, while I'm at it, that might be useful beyond just deleting and re-creating the accounts? Thanks again for all the advice.
I think the fact that you were able to setup mail in a new user account and smart mailbox worked pointed to either there was an issue with the mail in your account or something is corrupted in your user account in general.
But in that new user account, I had to set up a Mail as a "new" app.
In that instance back then, I created a new user account, opened Mail in it as a fresh unused app, and put in one bare account that I'd prepared for the purpose. And made a successful smart mailbox.
Taz, I'm five mailboxes of six into re-creating the Mail app from scratch. And so far, smart mailboxes are working fine!
OK, sixth account in, so that's the lot. And smart mailboxes work!
The pessimist in me is saying well, yes, they worked for a while the first time and then stopped, so try it for a bit and see how it goes. (So you may hear from me again.) But the optimist in me, usually the louder of the two, is cheering, and saying thanks so much for your patience and your clear advice, Taz.
Haven't posted for some time here, so what's the etiquette for officially thanking you? Can I thumbs-up or otherwise mark my thanks?