how to bypass virus scans?

Hello,
i have a problem with amavis that i hope someone can help me fix....

I run a mailserver from a box, with vhcs configured postfix + amavis +
spamassassin ...

Now ...
I have a fairly high rate of email sending (i manage a site of job
offers plus i have mailing lists, that makes a count of more than 30k
mails a day outgoing), and i find that when those sends starts the
clamav takes up more than 6 cpu load (altough i have a fairly high end
server)...

So i was thinking about disabling the check for outgoing mail (all of
it are sent trough php sendmail function).

Any way to do that? I read around quite a bit but still have not found
any clues ...

Thanks in advance!