Normally high server load is caused by the anti-virus backup scanner (ClamAV-clamscan) when the primary scanner (ClamAV-clamd) is down. By default Amavis will fall back to the backup scanner when the primary scanner is down or having problems. Note that the clamscan binary is NOT the same as clamd. Clamd is the highly efficient daemon version of ClamAV while clamscan is the inefficient non-demonized version. ClamAV-clamscan is not suitable as a backup scanner on busy servers because the ClamAV signatures have to be loaded into memory for every scan therefor we recommend that admins set the anti-virus scanners backup to none to disable the backup scanner if you are sending large amounts of mail.
Anti-virus scanners -
The list of primary anti-virus scanners (comma separated).
Anti-virus scanners backup -
The list of backup anti-virus scanners to use if the primary scanners fail (comma separated).