The general rule is...........delivering mail to the US Post Office is deemed to be filing, for any IRS purposes.  (One notable exception is the TD F 90-22.1 Foreign bank accounts).

The problem is always the proof of delivery to USPC...........and then of course making sure IRS eventually gets it, so business can proceed.

We mail nearly everything to IRS by certified mail, return receipt..................did you know that the USPC has NO obligation to track those mailings??  In the last five years, I bet the USPC has "lost" at least 6 pieces of mail we have sent to IRS by certified, return receipt.  

However, since we very particularly retain and safeguard the "white" certified mail receipts, we have proof of delivery.  (We mark the receipts, pay the postal fees by credit card, etc etc just to make sure we can "re-create" the delivery if need be.)

Retaining those white receipts has saved untold dollars of late filing fees, etc etc for the lost pieces, and also for others when the returns do not get processed at IRS Center for a few days.

 Recent court case from CCH this AM----attorney did it right, and since it was at Tax Court level, it was most likely a very valuable save-----------------********************

A married couple established that the Tax Court had jurisdiction to redetermine the IRS's determination of a tax deficiency because their attorney mailed their petition to the Tax Court within the 90-day period after the IRS mailed a notice of deficiency to them. The court received the petition late because the U.S. Postal Service (USPS) severely damaged the envelope in which the petition was originally mailed, rendering it undeliverable. Almost three months after the expiration of the 90-day period, the USPS returned the envelope to the attorney in severely damaged condition with the right half of the envelope, where the postmark typically appears, completely torn off. The attorney then promptly mailed a new copy of the petition along with the original damaged envelope and its contents to the court.

To establish that he had timely mailed the petition, the attorney submitted an affidavit stating that he had mailed the petition in a properly addressed envelope with a certified mailing label and paid the proper postage. He also submitted his postage log, detailing the date of mailing, the client's name, a description of the item mailed, the method of postage and the shipping cost. As further support, the co-owner of the mailing store from which the petition was mailed, submitted an affidavit stating that the USPS picked up the envelope on the same day the attorney left it. Thus, the "timely mailed is timely filed" rule applied, and the Tax Court had proper jurisdiction over the matter.

M.W. Van Brunt, TC Memo. 2010-220, Dec. 58,354(M)