Hi ! There is a simple solution, but i don't know if it is the most correct one or even secure
I guess it is ok (speaking of security) because if the zip (or rar) file is a virus or some sort of spam, it would come without password.
stop the zimbra services (as zimbra user)
edit the file
and change the following line (line 273) from:
%%uncomment VAR:zimbraVirusBlockEncryptedArchive%%ArchiveBlockEncrypted yes
save the file and start the zimbra services again
%%uncomment VAR:zimbraVirusBlockEncryptedArchive%%ArchiveBlockEncrypted no
And it should work
You can try by creating a zip file with password and send it over to you...
if it goes ok, then it's working !!