how to control ISP email retention on iPhone 13, iPad Air 4

I've noticed that my phone currently has only 2 weeks of email from my ISP email account downloaded, even though I have had the email account setup for many years on my phone. I don't recall setting any retention for the account, so it puzzles me why Apple Mail doesn't have all the messages from the email account.

This is not a theoretical exercise. This came to light because I was searching for an invitation to an appointment that was sent in early February, but it is not in my mailbag on my iPhone, so I had the impression that it was not sent to me. I was wrong; it was, it simply has been deleted from my mailbag, but not by me.

Note: This issue is NOT about email messages that I have deleted.

Thanks,

David