The remote host is vulnerable to a Denial of Service (DoS) attack.
The remote SMTP server is running FTGate Pro. There is a flaw in some versions of this software that may allow an attacker to disable it remotely by sending a too long argument to the MAIL FROM and RCPT TO SMTP commands. In addition, there are flaws that would allow an attacker to execute arbitrary code on the vulnerable system.
Upgrade or patch according to vendor recommendations.