Dovecot setup - authentication not discoverable

Version 1.760

I have set up Dovecot mail. All settings are defaults. No SSL.
I created a user in one of the virtual domains.
The user can login on IMAP or POP3 using telnet. This is the resulting line from

Oct 22 03:37:50 solar dovecot: imap-login: Login: user=<rmenger.mydomain>, method=PLAIN, rip=xxx.6.147.14, lip=xxx.170.86.128, mpid=10377, session=

So the user name, password, protocol and authentication are correct.

However when I try to set up the same user in Thunderbird, Thunderbird is not able to detect the server settings. As you know, Thunderbird insists on checking the password before it proceeds. It says it cannot verify the password, and this is what the log shows.

Oct 22 03:43:53 solar dovecot: imap-login: Aborted login (no auth attempts in 1 secs): user=<>, rip=xxx.6.147.14, lip=xxx.170.86.128, session=

Note the “user=<>” for the user name.

Now this is NOT a Thunderbird problem. When I follow exactly the same sequence, but try to set up the account on a server running Virtualmin 1.740, Thunderbird detects the settings just fine and authenticates the user.

The server running 1.740 has default configuration for Dovecot as well. At least, in all config screens for dovecot the settings are identical.