antivirus software - any suggestions?

Hi,
we are atm evaluating antivirus software on our mx gateway.
It's running Domino 6.53.

So far we have used Norton AV but the linux version (older - not on par 
with windows) let's you update the definitions only once a day. This is 
much too infrequent and has changed in the newer windows version. So 
Norton is out.
Then we have tested Trendmicro Scanmail 3.
We have received the latest version for testing but it has failed 
miserably and is in our opinion really dangerous.
It finds only about 50% of viruses. We can reproduce it finds the same 
virus in one mail, the next (excatly the same) email is ignored!

Any suggestions?
Thanks
Rainer