When away from your home network (also known as roaming), you can set the Mail application to update your Inbox only when you request it. This will minimize the amount of data you use and thus the cost for you.
You can disable autosync and background sync the following way:
- To disable autosync select Accounts > Your mailbox > (Sync Schedule >) Update Inbox > Manually. Do same for all your mailboxes.
- To disable background sync select Settings > Background apps > Mail > No updates (or Wi-Fi only).
Note: You could also block the mobile data connection when roaming by selecting Settings > Mobile Data (or Dual SIM) > Data Roaming > Don't roam.