Groups not resolving - final solution

Let's go through this without any assumptions (or try to, anyways).

Problem
Users in the parent domain send an email to a group (whose users are all in the parent domain). Email does not come through, no errors, no indication to user that the email has failed. If the email is also addressed to individuals (internal or external), those individuals receive the email.

On the Exchange side, the message log for an email sent only to a group stops at the 'Categorizer' step. For an email sent to a group and individuals the log completes for the individuals, but the group is not logged past the Categorizer stage.

Environment
Exchange 2003, AD at the 2008 functional level. Exchange 2010 domain preparation has been performed. Clients using WindowsXP/7 and Outlook 2007/2010. Exchange server is hosted in the parent domain. Child domain connects to that server.

Exchange 2003 machine has Directory Access set to automatically discover DCs, and sees all the DCs in the domain (5 child domain, 6 parent domain). No stale entries.

The groups in question (thus far) only contain parent domain users, and no stale user entries in these groups. At the same time that userA will try and fail sending an email to groupA, userB can successfully send an email to groupA.

Experiment
Using the 'reconnect' button in Outlook Connection Status, I will attempt to send to a group when connected to each separate DC. The object is to see if a single DC connection is causing issues.

Tested sending to DistList-Universal, SecurityList-Universal, DistList-Global, SecurityList-Global.

Test1: 8:55am
This test was just sending to DistList-Universal, DistList-Global, SecList-Universal, SecList-Global. Below are the message log results:

DC1: Only DistList-Global was stopped at Categorizer, other three successfully sent
At this point I went on a hunch and disregarded the other three, focused only on DistList-Global.
DC4: Success.
DC5: Success.
DC6: Success.
DC1: Success.

Test2: 9:24am
This test was just sending to DistList-Global. Below are the message log results:
DC3 – Stopped at Categorizer
DC4 – Success
DC5 – Success
DC1 – Success
DC6 – Success
DC2 – Success
DC3 – Success
DC4 – Success

Test3: 9:35am
This test was just sending to DistList-Global. Below are the message log results:
DC1 – Stopped at Categorizer
DC2 – Success
DC1 - Success

Test4: 9:45am
Tested sending only to DistList-Universal.
Success for all DCs twice around.

After setting logging for the Categorizer to 7 in the registry, discovered the following:

(missing the screencap...here's the text)
MSExchangeTransport
Categorizer
Event ID 6009
Categorizer encountered a hard error while processing a message. While processing user 'SMTP:groupemailaddress@domain.com', the function 'CCatExpandableRecpi::HrExpandAttribute' called 'plUTF8->BeginUTF8AttributeEnumeration' which returned the error code '0xc0040550' (). ( f:\tisp2\transmt\src\phatq\cat\src\ccatrecip.cpp@3048 )

Possibilities
I Googled the error specifics and came up with two options:

• Errors could be due to Symantec Mail Security – Heuristic Spam. Other suggestions turn up similar recommendations against various AV/Anti-Spam.
  

• Another suggestion is that it could be related only to Global groups, and to convert the groups to Universal. “…issue can occur if mails are sent to Global Distribution Groups in a multiple domain environment…” This was confirmed by me testing sending only to a Universal group - I could not reproduce the issue.

I conferred with our team, and they confirmed that the GFI Mail Essentials and Mail Security software we are using is out of date and can be removed. We have an external check for spam/AV done by MXlogic. They also agreed that the proposed root cause is most likely correct.

I will post in the comments as to whether or not that actually resolved it. We do have the Universal group thing to fall back on. We are attempting the GFI option first because it needs removing anyways, and moving from Global to Universal groups is a much more drastic change.

What I learned

1. Never base an investigation on assumption. Ever.
2. We do not have AD sites configured in our environment.
3. We have out-dated GFI software on the mail server.
4. AV/Anti-Spam can cause weird random stuff to happen to Exchange.
5. A lot about how Outlook works (Connection Status, Directory Access round-robin).
6. A lot about how Exchange works (Categorizer, Directory Access tab).
7. That logging can go to '7' via the registry (and that Exchange has a lot of logging options).
   

Background Notes
Categorizer
The Categorizer is responsible looks up the proxyAddresses attribute in AD for any recipients of the message. If that list includes a distribution group, it expands the group (unless an expansion server is configured). Currently the group in question has ‘any server in the organization’ set (Exchange 2003 machine plus the two new Exchange 2010 boxes).


Sources
SMTP Categorizer: http://technet.microsoft.com/en-us/library/aa998977%28EXCHG.65%29.aspx
Categorizer logging: http://msexchangeteam.com/archive/2006/06/23/428114.aspx
Group conversion solution: http://social.technet.microsoft.com/Forums/en-US/exchangesvrtransport/thread/6a453b9c-7b77-4f10-8567-87d6118fdec3
AD integration Ex2003: http://blogs.msdn.com/b/douggowans/archive/2007/01/04/ad-integration-from-2003-to-2007.aspx