Cisco PIX changes the command letters to 'X' before passing them to the real SMTP server. Exim example: 550 EXPN not available to (foo.bar.com) [192.168.0.1] Exim example: 550 EXPN not available to evil.com (foo.bar.com) [192.168.0.1] example: 550 lists are confidential 502 command is not active Lotus Domino example: 550 Unable to find list 'list'.