Intermittent IMAP Login failures - about 25% fail

[Mobile Phone]

New server & just added three domains:
now I see
"dovecot: imap-login: Aborted login (auth failed, 1 attempts in 5 secs):
user=<username.companyname>, method=PLAIN, rip=1.1.1.1, lip=99.99.99.99,
TLS, session=<kw6Y2NYviQBex5Un>"
Plus all Outlook users keep being bounced and finally get in.
I am going to kill all the users soon...

Not too many files open, nor bandwidth, nor load.

Any light/help appreciated.



root at brazil:/var/log# dovecot --version
2.2.9
root at brazil:/var/log# dovecot -n
# 2.2.9: /etc/dovecot/dovecot.conf
# OS: Linux 3.19.0-58-generic x86_64 Ubuntu 14.04.4 LTS
auth_debug_passwords = yes
auth_mechanisms = plain login
mail_location = maildir:~/Maildir
namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix =
}
passdb {
  driver = pam
}
protocols = imap pop3 pop3
ssl_cert = </etc/dovecot/dovecot.pem
ssl_key = </etc/dovecot/private/dovecot.pem
userdb {
  driver = passwd
}


Thanks