You can disable virus scanning for ALL outgoing emails sent by your users with below steps:
*) Open /etc/amavis/conf.d/5-user, find below line:
$policy_bank{'MYUSERS'} = {
...
#bypass_virus_checks_maps => [1],
...
}*) Remove '#' in line "bypass_virus_checks_maps => [1],", then restart Amavisd service.
That's all.