I think Tmobile has the cheapest plan and also lets you turn the unlimited international email on and off. You can even do it within the month and only pay for those days or weeks.
$39.99 for unlimited email and data and $19.99 for the unlimited international email when you have it activated.
But their network coverage is not nearly as good as the others so check the coverage where you live and work. If you're always near a wifi spot, you can get email over UMA instead.