Stop amavisd:
su - zimbra zmamavisdctl stop exit
Edit fstab:
vim /etc/fstab
Add:
# Mount amavis temp to RAM disk tmpfs /opt/zimbra/data/amavisd/tmp tmpfs defaults,noexec,nodev,nosuid,size=1024m,mode=750,uid=zimbra,gid=zimbra 0 0
Restart amavisd:
su - zimbra zmamavisdctl start