I've run into a problem when my mdaemon server sends email to a domain whose MX are at Postini. After receiving a single 451 because
451 Recipients not all at same mail host - psmtp
a message that is supposed to be delivered to 2 people, gets delivered only to the first one. Is there a way to deal with this in MDaemon? Queue the message and resend it to the rest of the recipients at a later time?
The problem is not with Postini. It is with MDaemon. According to the release notes, version 6.8.5 was released οn September 12, 2003. The problem seems to have been fixed in version 8.12 which was released on September 7, 2005.
If the second recipient is really not receiving the email (and not just delayed, received later), it sounds like a problem at Postini.
http://www.google.com/support/appsecurity/bin/answer.py?hl=en&answer=139512