TJH - 5/12/2026 08:24 The postmark should be good enough when it gets there. That's if they have postmarked it yet. My tax guy had us using a Certificate of Mailing from the post office. You can print one out online, fill it out and they time stamp it. Costs a couple bucks. If the envelope doesn't show up you have proof it was mailed, thus getting any penalty voided.