In iOS Mail, you can choose. For Exchange accounts, Mail can automatically download only the most recent messages, mail up to a month old, or all mail. If you're wondering where all your old emails have gone, it may be the default of synchronizing three days back that's hiding them.
Make iPhone Mail Sync More, All or Less Mail
To pick how many days of recent mail to synchronize with an Exchange account in iPhone Mail:
- Tap Settings on the iPhone Home screen.
- Select Mail, Contacts, Calendars.
- Tap the desired Exchange account under Accounts.
- Now tap Mail Days to Sync.
- Pick how many recent days' mail you want to be sent to iPhone Mail automatically.
- Choose No Limit to synchronize all mail.
- Note that there is no way to see or search messages older than the synchronization limit.
- Press the Home button.