I've had to test some connectivity issues with pop and imap clients without having access to their account info.
"/etc/virtual/{domain.tld}/passwd" file holds their user and md5 password hash. Make sure to backup this file first.
Generate a new md5-based password hash:
openssl passwd -1
Edit "/etc/virtual/{domain.tld}/passwd" file replacing the password hash with the above.
Test login to mail.
Once debugging is finished, restore the file back.