10.3.4 Firewall Telnet Checking May Inhibit SMTP Email
Certain firewall products, such as those from Symantec or Norton, check for telnet access and disable any such connection attempts. An SMTP connection on standard SMTP socket port 25 made by the DESTA Director to an SMTP email server may be interpreted by such a firewall as a telnet connection attempt, and the email may be blocked. In this case, you may see a message in the Director's log file such as:
SMTP< 421 mynode.mysite.com configuration errorSee Section 2.5 "Log Files" for the location of the Director log
As a workaround, disable the telnet checking feature of the firewall.