Mail USPS - which service?

[planetm69]

I'm mailing my past due 2020 tax return and need to know best way to send it. I read Certified with Return Receipt is best way but then somewhere else says if the IRS can't sign, it will be returned. Can you please tell me best way to mail through USPS?

Thank you. 

[CatinaT1]

This is up to you and your level of comfort.