Postfix/SMTP error

Hi, I am having trouble sending email from my mail server to .Mac accounts. This only affects .Mac accounts - I can send/receive mail to/from AOL, comcast.net, other businesses, etc. Here is the error from my SMTP log: Feb 9 12:46:31 myserver postfix/smtp[6975]: A6BA723659: to=<someone@mac.com>, relay=smtp-mx.mac.com[17.250.248.49], delay=71875, status=deferred (host smtp-mx.mac.com[17.250.248.49] said: 451 4.1.8 Domain of sender address me@mydomain.com does not resolve (in reply to MAIL FROM command)) I have done a DNSReport on my domain, MX record, A record etc. and all checks out fine. As I said, this error only happens when sending to .Mac email accounts (from any of the mail servers on my Xserve). Any ideas? Thanks in advance, Cory