This indicates an attempt to exploit a Denial of Service vulnerability in SASL authentication component of Dovecot server.
The vulnerability is due to a validation error when the vulnerable software handles a crafted username when processing SASL authentication if auth-policy component has been activated. A remote attacker may be able to exploit this to cause a denial of service condition on the affected system.
Dovecot Dovecot 2.2.25 through 188.8.131.52
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.