U.S. citizens and resident aliens who file calendar year individual returns are required to file a tax return by April 17, 2018. However, if you’re living and working overseas on the due date, you receive an automatic extension of two months (generally until June 15 for calendar year filers) if you attach a statement to your income tax return indicating that you're a person described in Treasury regulation 1.6081-5(a). Your return must be postmarked by June 15, 2018. You don't need to file Form 4868, Application for Automatic Extension of Time to File U.S. Individual Income Tax Return, in order to receive the two-month extension - you need to file Form 4868 only if you need additional time to file beyond the two months.
Consider using a designated delivery service to ensure the delivery of your international correspondence. An individual federal income tax return properly mailed from a foreign country or sent via a designated delivery service that's officially postmarked or dated in the foreign country or by the delivery service on or before the due date is considered to have been filed timely upon receipt by the IRS. If the last date for filing falls on a Saturday, Sunday, or legal holiday in the District of Columbia, you must file your return by the next day which isn't a Saturday, Sunday, or legal holiday in the District of Columbia. Keep your proof of international postage.