How to configure Zimbra to download clamav antivirus updates using a proxy.
Start by editing this file.

/opt/zimbra/conf/freshclam.conf.in

Then look for the following section.

#HTTPProxyServer myproxy.com
#HTTPProxyPort 1234
#HTTPProxyUsername myusername
#HTTPProxyPassword mypass

Uncomment at least the first 2 configuration statement. If the proxy requires authentication, uncomment the last 2 statements as well. Also specify the correct proxy server. This can be a hostname or IP address.
After saving the change, make sure to restart Zimbra

zmcontrol restart

Now wait for freshclam to run and you’ll find the updated files in the following directory.

zimbra@mail:~$ cd /opt/zimbra/data/clamav/db/
zimbra@mail:~/data/clamav/db$ ls -altr
total 121156
-rw-r----- 1 zimbra zimbra 109143933 Sep 22 10:45 main.cvd
-rw-r----- 1 zimbra zimbra  14809239 Sep 22 10:45 daily.cvd
-rw-r----- 1 zimbra zimbra     86357 Sep 22 10:45 bytecode.cvd
drwxr-xr-x 4 zimbra zimbra      4096 Sep 22 10:45 ..
-rw------- 1 zimbra zimbra        52 Sep 22 10:45 mirrors.dat
drwxr-xr-x 2 zimbra zimbra      4096 Sep 22 10:45 .
zimbra@mail:~/data/clamav/db$

You want to update SpamAssassin using a proxy?