Also note that default only 1 away message is sent per incoming message address over a specified time.
zmprov ma
user@domain.com zimbraPrefOutOfOfficeCacheDuration 3d
(you could also modify it on the COS)
Thus duplicate away messages will not be sent to the same user in the given interval of 3 days. On day 4, if they send that account an email, they would again get back one notification, then no notifications for another day.
Setting to 0 would send an out-of-office notification in response to every email received (I'd avoid that).
Plus, if you have user A with an away message up, user B sends an email, and immediately puts up their away message (with the attribute set to 0) you could technically get a loop.
I prefer 3d or every week 5/7d depending upon how long the user is gone. ie: If they're typically gone a month, then every week.
The other attributes:
zimbraPrefOutOfOfficeDirectAddress
zimbraPrefOutOfOfficeReply
zimbraPrefOutOfOfficeReplyEnabled
zimbraPrefOutOfOfficeFromDate
zimbraPrefOutOfOfficeUntilDate