Dovecot IMAP authentication failed

Firstly I assume that user celestine (with password 100995) is set up correctly in your system (simply check if a login is successful with no further errors / warnings).

According to your dovecot.conf, you set up your server authentication process using Lookup databases with passwd. That's fine as User Database, but with passwd as Password Database you can't get the user's password on Debian, as stated in the official Dovecot documentation:

hxxp://wiki2.dovecot.org/AuthDatabase/Passwd :

Most commonly used as a user database. Many systems use shadow passwords nowadays so it doesn't usually work as a password database.

You can work around this problem with

passdb {
  driver = shadow
}

Another possiblity would be using passdb with Passwd-file, but I recommend you set up your Dovecot authenticate system using PAM.

Good luck, Megodin