libs/comverp.c in Courier MTA before 0.53.2 allows attackers to cause a denial of service (CPU consumption) via unknown vectors involving usernames that contain the "=" (equals) character, which is not properly handled during encoding.
Double Precision Courier e-mail MTA allows remote attackers to cause a denial of service (CPU consumption) via a message with an extremely large or negative value for the year, which causes a tight loop.