For your possible interest, here is a post from AMUG -- the Arizona Mac User Group here in the Valley of the Sun. In addition to discussing email clients there are links on the website that discuss a variety of other Mac-related topics. I stopped using Apple's Mail client many years ago, went to Gmail, and never looked back. On the AMUG Forum there are frequent postings of members having problems with Apple's Mail program. That's one reason I stay away from it, but many people do use and enjoy Apple's Mail Program.
Jim Hamm
The link below is from Randy Singer. Randy is an attorney and has for many years run a well respected online group for lawyers. In this email from him he talks about the email issues that some of you have had with Catalina. You are not alone. He has gathered information on a number of alternative email clients that you might want to explore....Alex (Secretary of AMUG)
"I just put up a new Web site. Apple's Mail under Catalina has a nasty bug that causes it to lose data. Some users have been looking for an alternative. I created a page listing all of the available Macintosh e-mail programs!"
Macintosh Email Software
http://www.macattorney.com/mail.html