I followed these instructions
1. Integrating avast antivirus
Open the AMaViS configuration file (e.g. /etc/amavis/conf.d/50-user)
and insert the following lines into the file:
@av_scanners = (
###
http://www.avast.com['Avast', '/bin/scan', '{}',
);
2. Enabling virus scanning
Then open the AMaViS content filter configuration file (e.g.
/etc/amavis/conf.d/15-content_filter_mode) and enable antivirus checking
mode by uncommenting the ’bypass virus checks’ lines.