Sync in email means that it will compare all the folders in your email client/app with all the folders on email servers and see whether it needs to import, delete, or move messages to other folders, and whether or not it needs to add or delete and folders you've created.

Should I have sync on or off?

Turning off auto syncing for Google's services will save some battery life. In the background, Google's services talk and sync up to the cloud. ... If you turn off the location settings off many apps will not triangulate on your location using the in-built GPS in the phone, which sips more power and battery life.

What is the use of email sync?

Mail Sync is a powerful tool that allows you to connect your inbox to SharpSpring. Once enabled, any email you send through that inbox—and any Smart Mail that you send out of SharpSpring will be recorded. This way, you can keep track of you and your team's emails with your leads.