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
Resolution: 400-series responses are transient (non-fatal). A 451 error defers the message back to the sending MTA, which should queue the message and resend it at a later time. The message security service does this to handle the case where a message is sent to two different users from the same domain who are configured under different mail servers.
Because this is a deferral, there should only be a momentary delay before the message is delivered. An MTA should not bounce a message because it received a single 400-series response to a message transfer.
If the sending MTA drops the message, look closely at either the sending server's SMTP logs or any bounce messages received. Either the sending MTA doesn't understand that a 400-series response is a transient (non-fatal) error, or there will be a 500-series response nested in with the 451 errors in the bounce message. The 500-series response is a permanent fatal error which would cause an email to bounce.
