Many of the RTOs described in TFA already have extended hours on April 15 where you can drop it off up to midnight, and it would be post marked as the 15th even though the postmark stamping would be after midnight.
Last year I was hit with a failure-to-pay penalty, even though I mailed the check on the deadline. When I spoke to an IRS agent, they claimed that the payment needed to be received, not postmarked, by April 15. Never heard of that before. He was kind enough to remove the penalty because it was a first time offense.
It's strange to me that they list them in that order... making me think they meant "addressed, stamped, and deposited", since postmarking clearly occurs later, after the others.
