The Electronic Postmark is a timestamp of when your return was transmitted.
IF you used the "online" software
and
IF you log into the exact same Online account you used to file 2020 taxes
Then, you can download a NEW PDF copy of the 2020 tax return from your online account, open that PDF and look for the sheet called "Postmark Report"
...that sheet contains both the submission time/date, and the acceptance date
