I use Postini for incoming only.  Outgoing mail is handled by SMTP on my corporate mail server but I too am having the same problem.  E-mail to clients that have worked for years is now getting blocked.  It doesn't matter if it is a new message or a reply.  My mail server is as clean as a whistle.  It is behind a firewall and only has 20 users.  I am not on ANY blacklists, however I cannot send critical business e-mail.  I cannot send to AT&T for example and it is impossible to get an end-user at ATT Corporate to figure out who is in charge of their mail server and there is no information in the bounce that would help with getting unblocked.  This is ridiculous!

Just found this on the McAfee Saas web site in regards to blocked messages: 

Forward the message with the original internet headers to falsepositives@mcafeesaas.com.

Did as suggested and that message got bounced too!  What a shock......

Anyone have a contact number for these people?

Thanks,

Kevin