Yes, the server was updated recently (not by me). I restarted all the services, I even re-installed Postfix and Dovecot.
The /usr/lib/sasl2/smtpd.conf file had the following line:
The output for "saslauthd -v" is:
authentication mechanisms: getpwent kerberos5 pam rimap shadow ldap
I tried adding mech_list: plain login, but nothing happened.
Thank you for your help!