Tested on Zimbra 8.7.7

You find the following in the /opt/zimbra/log/freshclam.log

ClamAV update process started at Mon May 15 09:27:11 2017
main.cvd is up to date (version: 57, sigs: 4218790, f-level: 60, builder: amishhammer)
WARNING: getpatch: Can't download daily-23387.cdiff from database.clamav.net
WARNING: getpatch: Can't download daily-23387.cdiff from database.clamav.net
WARNING: getpatch: Can't download daily-23387.cdiff from database.clamav.net
WARNING: Incremental update failed, trying to download daily.cvd
WARNING: Can't download daily.cvd from database.clamav.net
Trying again in 5 secs...
ClamAV update process started at Mon May 15 09:27:28 2017
main.cvd is up to date (version: 57, sigs: 4218790, f-level: 60, builder: amishhammer)
WARNING: getpatch: Can't download daily-23387.cdiff from database.clamav.net
WARNING: getpatch: Can't download daily-23387.cdiff from database.clamav.net
WARNING: getpatch: Can't download daily-23387.cdiff from database.clamav.net
WARNING: Incremental update failed, trying to download daily.cvd
WARNING: Can't download daily.cvd from database.clamav.net
Trying again in 5 secs...
ClamAV update process started at Mon May 15 09:27:39 2017
main.cvd is up to date (version: 57, sigs: 4218790, f-level: 60, builder: amishhammer)
WARNING: getpatch: Can't download daily-23387.cdiff from database.clamav.net
WARNING: getpatch: Can't download daily-23387.cdiff from database.clamav.net
ERROR: getpatch: Can't download daily-23387.cdiff from database.clamav.net
WARNING: Incremental update failed, trying to download daily.cvd
ERROR: Can't download daily.cvd from database.clamav.net
Giving up on database.clamav.net...
Update failed. Your network may be down or none of the mirrors listed in /opt/zimbra/conf/freshclam.conf is working. Check http://www.clamav.net/doc/mirrors-faq.html for possible reasons.

To clean it up

su zimbra
mkdir /tmp/oldclam
mv /opt/zimbra/data/clamav/db/* /tmp/oldclam
freshclam --config-file=/opt/zimbra/conf/freshclam.conf -v

after it is complete, review the freshclam.log for any other errors