Updates to meetings coming back as undeliverable

Users in our organization are reporting an issue when sending meeting
updates. The first time they send the meeting request to internal employees
or outside customers, everything is fine and the meeting schedules without a
problem. If the organizer needs to make a modification to the meeting, say
the time, an undeliverable report will be sent to them that it did not reach
all recipients, but only for outside email addresses. The undeliverable does
not come back right away. The email appears to sit in the queue until it
times out. The undeliverable received is "could not deliver the message in
the time limit specified. 4.4.7". So to the organizer, they think everything
has been sent out and updated without any problems because they don't receive
any undeliverable right away. We are running Exchange 2003 and all users
have Office 2007. Any help would be appreciated.

I have looked at the 4.7.7 error and read the details but the original request and any emails to the individuals on the request go through fine.