Notifications for emails are coming through Cellular Data > System Services > Exchange Accounts, which has no toggle to switch of cellular data for.
That means when toggling of cell data for mail, it means you can't receive the new mail your in your inbox but will still get notifications for them. I haven't figured out how to disable notifications
Since notifications use very low data, you'll be safe.
If you're on T-Mobile NL, you're safe anyway, they don't allow roaming unless you buy a Travel and Surf pass for roaming.