According to the domain host, DKIM, DMARC and SPF records were missing from both relevant domains/accounts, and installed them. I believe that they'd put them in a couple of years back, but that's what tech support said. About two hours later - time to propagate, I guess - everything was (and, blessedly, remains) working as before.

 ~ OOPS! ~

Well, it all worked perfectly for about a day, then fell apart again. So any help would be most welcome. Host tech support is friendly and engaged, but seems to think that the problem must be at my end. And they may well be right - I'm no IT expert.

I have about seven e-mail accounts - real mailboxes - on one shared-hosting domain, and one on a different domain with the same host. These accounts had multiple aliases/identities. Also have seven old AOL accounts that are still useful. I had Thunderbird set up with one SMTP server for each of the three domains. This has worked perfectly for years.

I am now able to get only one or the other hosted SMTP setup working, not at all for the other, and can't reach AOL through a separate SMTP setup using their values. With the domain that does work, I can use aliases, but only with the true domain name. (Before, it could be another TLD that I also own, but use only for receiving and not sending.)

Getting several different kinds of login errors. I did not knowingly change anything, and neither Thunderbird nor Windows has updated AFAIK.

Was I doing it wrong and just got lucky before, has Thunderbird somehow changed, could something else be wrong?

Thanks for any help!

Two things.

1. POP is an incoming only protocol. you will be sending using SMTP and serching for POP issues will certainly not get you much in the way of useful information.

2. Every account needs it's own SMTP server. This is especially so with AOL, but your host many have similar rules. Basically if the email address associated with the signon used is not the one being used as the from, the SMTP servers will refuse to relay the mail. It is a spam protection and faking protection that has become almost universal in the past 10 years or so.

Thanks for that, Matt. Realized I'd mistakenly written "POP" in subject line, but couldn't then edit it out.

It's interesting that this all used to work with just three SMTP servers listed in "outgoing servers" - one for each domain. Perhaps the host changed rules a few days ago, for the kind of security reasons you mentioned. "Official" instructions say:

  https://support.mozilla.org/en-US/kb/manual-account-configuration

"Multiple email accounts can use the same outgoing SMTP server. Therefore, SMTP servers are created separately from email accounts. In the Account Settings dialog, in the left pane, select Outgoing Server (SMTP) from the bottom of the list. There may be an entry in the resulting list that was created when you entered your account details. Either edit that entry or add a new entry."

I did try adding separate SMTP server entries for several mail account. Problem is that Thunderbird seems to make its own choices of server-per-account, even swapping port numbers around. Is this normal?

Since I can receive mail just fine, I could change the setup to just one outgoing mailbox and SMTP server per domain, and try to set up all the needed e-mail addresses as aliases/identities. Really prefer it as it was, though!

What's mystifying is why it worked exactly as wanted with that simple setup for several years, then suddenly came unraveled. And the host asserts that they've changed nothing. Again, maybe I was just lucky before, and now have to do it properly.

Will report back if/when it's sorted. Again, thanks.

OK, did it the hard way. Deleted every account, all information at both (shared) server and Thunderbird end. To avoid a long Outgoing SMTP Server list, condensed a large number of accounts down to just three, with remainder as "Identities"/aliases. Lotta hours, but it seems to work at present.

Can no longer use Thunderbird's very handy "Customize From Address" feature. The "plus address" approach, about which I just learned, seems to work with some (but not all) mail providers, so that may help.

Better-informed friends advise that my domain host probably made some security change w/o telling customers; haven't found any other explanation. Maybe for the best, but would have liked to avoid anloss-of-connectivity emergency. Still think that Mozilla is THE BEST, and am very grateful to the community that supports us amateurs.

Thanks!