HTC One M9 - Manual (Page 154)

What is Smart Sync?
Smart Sync automatically extends the sync time the longer the Mail app is inactive. Set your email
account to Smart Sync when you don't need to check new email messages frequently. Smart Sync
helps save battery power.
If you want to receive email messages as they arrive, select another Peak time sync and Off-peak
sync schedule in the email account's Sync, Send & Receive settings.
154 Calendar and Email