This indicates an attack attempt against a stack-based buffer-overflow vulnerability in Atrium MERCUR IMAPD.
The vulnerability is due to the software's inability to properly check the bounds of user-supplied input. A remote attacker could execute arbitrary code on the system by sending a crafted Subscribe request to the service.
Atrium MERCUR Messaging 2005 SP4 and prior
Currently we are not aware of any officially supplied fix for this issue.