I reckon I’ve worked out what the problem is, but not why the problem is.
Although there are well over a hundred different domain accounts on the server, only three or four (well, I can only think of four) that are email-only, no website at all.
For those, I didn’t set up a www. A record in the DNS. Don’t need one if you don’t have a website, do you? They did all have a public/htdocs folder because I guessed that was needed to get a SSL certificate. And a simple index.htm that said something like “hello”
Well, all seemed to go according to plan, except that none of these four accounts was getting any incoming email. None at all. They could send, but not receive.
So when I realised the common factor, I added the www A DNS record and force recreated the website and the SSL. Made no difference.
So I moved their entire /srv directory away to a safe place, deleted their sites-enabled and sites-available records, re-ran sudo sympl-web-configure (just to stamp on the site’s grave)
Then sympl create web domain.co.uk nano /srv/domain.co.uk/public./htdocs.index.htm
Thensudo sympl-web-configure and sudo sympl-ssl
Add the maildir/username directory.
Send an email, and it arrives!
Copied the moved-way mailbox contents in and they seem to be OK again.
I’m pretty sure there is something fundamental wrong with the setup if there is no www A record.
I have a suspicion that the webmail directory wasn’t set up either, but I was too focused on getting their email fixed that I didn’t investigate properly. Sorry.
Later: remembered a fifth. Yup, no webmail before I fixed it.