Till, maybe I didn't explain myself clearly.
The usernames for ISPConfig and for the O.S. are like "domain.com_user1"
I did all that extra configuration so you can have you email client send/receive email using "email@example.com" as the username.
It's working fine from the email client, but I can't log in in Squirrelmail ("wrong username or password").
So, there are not usernames with the character '@'.
May 16 20:56:52 mail dovecot: imap-login: Aborted login: user=<_domain.com_user>, method=PLAIN, rip=::ffff:127.0.0.1, lip=::ffff:127.0.0.1, secured
As you can see, there is an extra '_' that is added at the beggining of the username. I don't know why this happens.
For what I've read in these forums, if I install dovecot and configure it for "firstname.lastname@example.org" usernames and then install squirrelmail, it should work without any modification.
Hope I was more clear this time.
Thanks for your help.