First steps in Dovecot; IMAP not working

[Moi]

Hello,

First of all, I'm sorry if you received this mail twice; I haven't received
it the first time so I think it was lost. Second attempt.

I've made some more tests and I still can't receive mails.
Outlook doesn't complain about anything, no error message; the mails that
are supposed to be received just won't appear.
In the mail logs, I find this line (other lines are irrelevant):
dovecot: imap-login: Aborted login (no auth attempts in 2 secs): user=<>
[...]

Is this a problem that looks familiar?

It's a bit clueless without an error message.
Any idea welcome.