If you're using an antivirus solution on your Exchange 5.5 server that utilizes the Microsoft antivirus API, the antivirus software scans only the mail that goes through the information store. The Microsoft antivirus API can't scan messages that transfer only over the Internet Mail Service.
The preferred solution to this problem is implementing software that scans SMTP mail, but not every IT department has the budget for multiple antivirus products. You can also reroute your SMTP mail through the Exchange information store to ensure that it's scanned. This allows software written to the Microsoft antivirus API to scan SMTP mail.
You can force SMTP mail through the information store by setting the following registry value to 1:
Value Name: RerouteViaStore
Data Type: Reg_Dword
Once you stop and restart the Microsoft Exchange Internet Mail Service, all SMTP mail will reroute through the information store.
Before you make this change, be aware that rerouting SMTP mail through the information store automatically enables both ResolveP2 and AddressRewrite parameters. For more information on the ResolveP2 and AddressRewrite parameters, take a look at Microsoft Knowledge Base articles 174755 and 166163.
Note: Before editing the registry, be sure to first back up the registry so you can restore it if something goes wrong.