Cisco PIX changes the command letters to 'X' before passing them to the real SMTP server. Exim example: 550 EXPN not available Exim example: 252 VRFY not available Exim example: 250 <root> is deliverable Exim example: 250 <root> is not deliverable 502 command is not active I've seen this on NTMail 4.30.0012, NTMail 5.06.0016 sendmail example: 504 <foo>: Recipient address rejected: need fully-qualified address Lotus Domino