The IRS has updated their list of private delivery services for the first time in over ten years. The update has added three FedEx and one UPS service to the list and removed five discontinued services that were previously acceptable for the timely mailing equals timely filing/paying rule (Sec. 7502).
There are only 15 delivery services that taxpayers can use (outside of the post office) that will satisfy the mailbox rule. These services are provided by FedEx and UPS only, the complete list can be found here. This change is effective as of May 6, 2015.