Mail to the IRS can go unanswered for months. Because the IRS is so understaffed, and having trouble processing all the received mail, you need to protect yourself when you send an important tax filing due by a specific deadline.
If you can file a document electronically, do so. If you must file a physical document with the IRS, don’t use regular U.S. mail, Priority Mail, or Express Mail.
When you mail a document with these methods, the IRS considers it filed on the postmark date, but only if the IRS receives it. What id the post office doesn’t deliver it or the IRS loses it? You will have no way of proving you sent it.
Don’t risk it. File physical documents by certified or registered U.S. mail. When you do this, the IRS considers the document filed on the postmark date whether or not the IRS receives it. Always keep your receipt.