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.