I have set up a postfix server in front of Exchange. Things seem to have been running fine then lately I got a report from two different domains that are rejecting emails from us. This is the message from the bounced emails: host mini.tetherinc.com[67.1xx.1xx.xx] said: 500 Syntax error, command unrecognized (in reply to DATA command) So I ssh'd into my postfix server and tried a manual telnet e-mail. This is what happens: root@antispam:~# telnet 67.1xx.1xx.xx 25 Trying 67.1xx.1xx.xx... Connected to 67.1xx.1xx.xx. Escape character is '^]'. 220 mini.tetherinc.com ESMTP Service ready HELO antispam.appart.com 250 Requested mail action okay, completed mail from: firstname.lastname@example.org 250 Requested mail action okay, completed rcpt to: email@example.com 250 Requested mail action okay, completed data 500 Syntax error, command unrecognized Any ideas on what could possibly be causing this? It is only two domains that have shown this error but they are very important domains that we need to be able to e-mail with no problems. Please help!!