Adding a disclaimer (altermime) or footer - Zimbra :: Wiki
That will allow you to insert a system-wide message to all emails. This is a feature that is supposed to be available in 6.0 (probably).
Mailscanner worked spectacular. I used it for many years with courier imap, along with mailwatch. Zimbra is now using Amavis rather than mailscanner, I'd prefer Mailscanner as it is more expandable, but Amavis gets the job done.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now