Bug 31292 - XMPP client connections timing out after 10 minutes
Workaround for 5.0.9 (this is fixed in 5.0.10) is to go into /opt/zimbra/conf/localconfig.xml and add a new entry for the 'xmpp_client_idle_timeout' key with a large value.
The value is in msec, the default is 600000 (10 minutes). Make it 100 minutes, or 1000 minutes to lessen the annoyance to users.
Note that raising this value will lower the rate at which the server recovers some memory when clients drop connections. Make sure you change it back after you upgrade to the fixed version.
su - zimbra
zmlocalconfig -e xmpp_client_idle_timeout=600000