How to disable smtp scanning for a server

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[S3@N]

We have a server which we use for communicating with our customers which sends out mailings of around 4000 emails periodically.  This is currently affecting the performance of our Endian machine and I would like to reduce the impact.  Currently mails are going through the Endian smtp proxy and I suspect that the main performance issue is the virus scanning (amavis) on all these emails - is there a way to disable virus scanning of emails from a particular server?   I can't see anything obvious.

Thanks,