user: Windows XP SP2, Office XP with Outlook, Exchange Server account
network: AD with Exchange Server for email
problem: User within network not receiving meeting invites from
internal network users, either singular meeting invites just to the
user or group meeting invites in which the user is part of the group.
So far, no issues with meeting invites from users outside of the
network.

so:
userA@network.com is not receiving invites from userB@network.com or
[anyuser]@network.com on a periodic basis.

userA@network.com still receives invites from people outside of the
network.

Nothing particular has been changed about our network or systems with
the small exception of patch deployments for the DST2007 change.

I'm tying a few test invites and message tracking but it's really
strange.

Especially since there are multiple instances where:
userC@network.com sends an invite internally on the exchange network
to userA@network.com, userB@network.com & userD@network.com (where any
user may be in a different physical office, but within the network)
and only userA does not receive the invite.  Yet any regular mail from
userC is delivered fine.

I've been searching for weeks, but I can't come up with what the issue
might be.

Any insight would be greatly appreciated.