no one can connect to their mail

Help please, clients are calling and are very angry.

No one is able to login to their email, via webmail (squirrelmail) or via imap (outlook). They are prompted that their password is incorrect. This is affecting multiple (perhaps all) domains. I am able to read their inbox via webmin > servers > read user mail.

Dovecot was updated last night via YUM, but I don't know if that has anything to do with this. I have turned off the firewall but have the same problem.

Help please! Where can I look, what more can I check?