amavisd-new ("a mail anti-virus system") is one of Zimbra's open source components, but I'd expect the path to be /opt/zimbra/amavisd-(version)/tmp (in 5.0.x, this moved to a subdirectory of data/). It's also possible someone that installed their own custom copy.
ls -ld /opt/local/var/amavis/tmp
Is it owned by user zimbra?
Is /opt/local symlinked to /opt/zimbra or vice versa?
In any case, old things in "tmp" most certainly can be purged... |