I used MailMate (https://freron.com/ https://freron.com/) before and I liked it. Very customizable and the search functionality is better than Apple Mail or Out

I used MailMate (https://freron.com/ https://freron.com/) before and I liked it. Very customizable and the search functionality is better than Apple Mail or Outlook. Has some tool extensions to allow it to interact with some other apps. It does not have a built in calendar though, so you will need to either get something else (there is an extension to allow MailMate to work with some calendars).