If you use Zimbra antivirus, and it doesn't recognize/block a given virus (IN EMAIL, of course), then you should submit it to ClamAV.
Clam AntiVirus
Note: if the virus is zipped, you might want to submit both the zipped and unzipped versions, although since
Bug 45607 - Suspect.Bredozip-zippwd-13 .zip virus not detected is fixed in ZCS 6.0.7, I believe that Zimbra should detect the virus in either form. (As a workaround until 6.0.7 is released, see the comments in the bug; see also
Is AntiVirus really working?)