you are correct about the new postmark date rule (takes effect today).

The silver lining is that most tax returns aren't due on April 15 - only the ones that owe taxes are. And you can still walk into the post office and have your mail hand-canceled, and that's the date that will count. But dropping it in the collection box does mean it can wait for a day or three before the post office formally processes it.

All the tax accounting websites have warnings about the new rule, if anyone is interested in more details.