It is not uncommon to find an architecture like the one below:
I was involved with such deployment a year ago with a local corporation. The back-end is Microsoft Exchange.
Basically, there are 3 layers of messaging filtering:
1. MessageLabs Anti-Spam/Anti-Virus Filtering (Hosted/Cloud version)
2. TrendMicro InterScan Messaging Security Suite (In-house)
3. Symantec Mail Security for Exchange (In-house)
The primary idea behind choosing different vendors at different layers is to ensure that most (if not all) illegitimate emails are caught.
This is what I call Total-Defense - you do not use the same key to lock all the doors in your house for convenience sake.
Yes, I do agree that liaising with 3 different vendors is a nightmare. But for the sake of security, this is inevitable.
Ok, MessageLabs is now under Symantec umbrella. Maybe it's time to switch to Goggle Postini Service, or even ProofPoint SaaS Email Security Solutions.
However, I am still not convinced with ProofPoint customer service support, especially in the APAC region.