This indicates an attack attempt against a vulnerability in the SMTP service in A-V Tronics InetServ SMTP service.
The vulnerability is due to the software's inability to properly handle malformed SMTP commands. A remote attacker may be able to exploit this condition to cause a denial of SMTP service.
A-V Tronics InetServ 3.23
Denial of service
Currently we are not aware of any patches supplied by the vendor for this issue.