To minimize your data cost, you should set the email sync to Manual. To do this, perform the following steps:
- Go to Home screen.
- Select Account.
- Select mailbox (email provider).
- Select Update Inbox (or Schedule sync > Update Inbox)
- Change to update inbox Manually.
To disable background sync select Settings > Background apps > Mail > No Updates or Wi-Fi only.
Note: To minimize the data cost, you can also use the wireless network (Wi-Fi) to retrieve emails whenever possible.