This indicates an attack attempt to exploit a Denial of Service Vulnerability in Dovecot.
A remote attacker could exploit this vulnerability by sending an AUTH command, and aborting it before receiving a reply. Successful exploitation will result in the termination of the submission-login process.
Dovecot 2.3.0 through 188.8.131.52
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.