Generally speaking it is sufficient that income tax returns are mailed to the IRS within the applicable filing dead-line. This is known as the "timely mailed is timely filed" rule. The timely mailed is timely filed rule also applies to returns sent from outside the US, as contained in IRS Rev. Rul. 2002-23.

The best advice regarding filing dead-lines is to mail income tax returns as soon as possible in advance of the filing dead-line, make sure to use the correct mailing address, and always use a delivery method that provides proof of delivery.