This indicates an attack attempt to exploit a Denial of Service Vulnerability in Dovecot.
The vulnerability is due to infinite loop in the submission-login and lmtp components when processing truncated command parameters. Successful exploitation will result in the application consuming 100% CPU and becoming unresponsive, leading to a denial-of-service condition.
Dovecot Dovecot 2.3.9 and prior
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.