Man, Oh Man!! I came across this dreading mail issue. Here is the problem description.
Users complained that they were not receiving emails sent to a Distribution List. Mails were sent from an external application to the Exchange DL.
When I track the mails in Exchange, I see the mails came into Exchange and got stuck in to "Categorizer". All I saw was the following three lines and that’s it.
Solution: Make the DL as Universal. I realized it resolved my issue. Then I read the DL has to be Universal to receive external (Internet) mails.
If that is not resolving your issue, it’s mostly your anti-virus software. It happen to me also. Few emails were mysteriously stopped at Categorizer and DL is already Universal.
After some research, I found it’s the Anti-Virus (Trend Micro ScanMail for Exchange) software that blocks the emails. I still don’t know why it was blocking. It must be the bug.
I contacted Trend Micro support. They suggested a patch for our Scanmail. I installed it on my Exchange server and so far I am not seeing the issue.
I also learned few things along the research.
– If mails are not delivered to users and it involves many different DLs, it’s probably the Global Catalog issue. Make sure your Global Catalog server is working normally. (check logging on to domain, do a "Entire directory" search in Active Directory Users and Computers). Follow the steps below to manually specify the Global Catalog servers on your Exchange server.
1. Open System Manager
2. Expand Administrative Groups -> Site -> Servers -> <Your Exchange Server>
3. Right click on your Exchange server and select Properties
4. Click to Open "Directory Access" tab
5. Click on "Show" Drop down box and select "Global Catalog Servers"
6. Uncheck "Automatically discover servers"
7. Add button to manually add a global catalog server
8. Click OK to close the dialog
– There could be "DL Expansion Server" issues on your Exchange organization. You may specify manually an appropriate DL Expansion server.
1. Open the properties of the Distribution List in Active Directory Users and Computers.
2. Select "Exchange Advanced"
3. Select "Expansion server" from the drop down list
4. Click OK to close the property dialog box