Welcome to the forums,
Account aggregation:
-4.5.x supports external POP accounts in the web-client (Not that you care - you're on v5!)
-5.0.x supports both external POP & IMAP accounts in the web-client (plus periodic checking - so you don't have to hit 'get mail')
Before you ask

When you want to set it automatic:
Quote:
su - zimbra
zmprov mds user@domain.com [Data source external IMAP/POP account name] zimbraDataSourcePollingInterval 900s
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zimbraDataSourceMinPollingInterval is usually 60s or 1m or 2m & zimbraFeatureMailPollingIntervalPreferenceEnabled should be set to TRUE
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 "", 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