Syncing email manually

[AC Question]

If I have the master sync button on(and all other email sync buttons on), will email sync in the background if it is set to manual...or do I have to open the email app? In other words, will email automatically sync even though it is set to manual and the app is not open? Thanks. Reason: I would like to turn on the VPN prior to retrieving emails...VPN on all the time is a battery drainer.