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major email problems

Buster, Stretch, Wheezy

I know this is probably the wrong place to ask, but DU is full of
knowledgeable folks...

This morning I quit receiving IMAP email (Dovecot, Thunderbird). I
installed and tried several other clients (iMail, Geary, Sylpheed) and
none worked. They said things like my AuthenticationMethod (PLAINTEXT)
was wrong -- I tried several others; none worked. They said my
userName/Password was wrong. I re-entered the password, still nothing.
But I could log into the server with the name/password.

Since several clients wouldn't work, I looked at Dovecot's config. I
couldn't find anything there that looked promising -- and besides, it
was fine yesterday.

I reinstalled Dovecot from a Buster mirror.. Same errors.

I tried 'telnet <srv> 143', and entering IMAP commands by hand. I got
some useful info: PLAINTEXT was disallowed on non TLS logins. It said
that right after I entered the userName, not when I entered the
password. It said the same thing when I did enter the password.

I couldn't find anything in the Dovecot config files that addressed
that, except a commented out line: "#disable_plaintext_auth = yes."
But Dovecot seems to be tossing PLAINTEXT anyway. I didn't try
deleting the line.

I tried a different computer (old server - Wheezy) thinking there
might have been a Dovecot update that made all my clients fail. Same

The mail log from yesterday shows successful logins all afternoon --
but this morning is shows authentication fails. So I tried changing
Dovecot's auth source from PAM to the shadow file. Nope.

I'm completely at a loss. I've been dealing with Dovecot for years
with no problems. Today, I swear some bit flipped all by itself. And I
can't find it.

If you have any idea of what I may have missed, I'd sure appreciate
hearing about it. And since I can't see the debian-user list, please
respond directly to me at ghe2001@xxxxxxxxx


Glenn English