I assume this is what happened:
ClamAV 0.94 EOL on ZCS 5.0.16 prior (discussion) and
ClamAV - Updating clamd for releases earlier than ZCS 5.0.16 - Zimbra :: Wiki
Also discussed on the support portal, if you have access:
https://support.zimbra.com/ClamAV0.94EOL
It's not a licensing issue.
As long as you watch for alerts in the Admin forum here and/or keep a subscription to the support portal rss feed, you should be alerted before anything like this happens. Also, if you keep your ZCS up to date with a reasonably current version, it shouldn't be an issue. The issue arose because ZCS 5.0.16 used a version of ClamAV that expired on 4/15/2010. The updated ClamAV was included in ZCS 5.0.18 which was released 7/8/09.