Where are my Sent emails stored?

Where exactly are my Sent emails stored?

Over many years of using MacMail I recently noticed that my sent emails only go as far back as 2020.

Is there a folder where they are all stored?

My email service provider says they do not store my sent emails.

When I was on ElCapitan, I had sent emails as far back as 2014 - but since upgrading to Monterey they now only go as far back as 2020.

Any help much appreciated, thank you.