Welcome to the forums (both of you),
su - zimbra
zmprov mc cosName zimbraDataSourceMinPollingInterval 5m
zmprov mc cosName zimbraDataSourcePollingInterval 15m
(cosName = default or other COS value)
You'll see a bunch of [ScheduledTask] [name=user@domain.com;mid=x;ds=name;] datasource - Import completed. in /opt/zimbra/log/mailbox.log
If that doesn't enable your external accounts to poll, try setting the account level:
zmprov ma
user@domain.com zimbraDataSourcePollingInterval 5m
Or at the individual data source level:
zmprov gds
user@domain.com
(spits out current configuration)
zmprov mds
user@domain.com [external IMAP/POP account data source name] zimbraDataSourcePollingInterval 15m
To cancel polling, set the attribute value to an empty string ('') or 0. Keep in mind that zimbraDataSourcePollingInterval gets inherited from the COS > to the account > to the data source. If a child object sets the value to '' null, it inherits the parent's value - if set to 0, it's turned off.
You can also vote for:
Bug 22114 - Expose external account polling interval in UI