The postmark on the envelope is used for your date of filing when received by the IRS. So long as it postmarked on or before April 15, 2024 you will be fine.
I highly suggest that you use a tracking service from the Post Office so that you will not only get receipt of when received by the USPS but will be notified when the tax return is received by the IRS. Keep your receipt and notification with your tax records in case there is ever a need to prove your mailing.
