Wilbur’s list is hard to argue against. 3 email apps for Mac, iPhone, and iPad.
If you don’t have many email accounts and don’t get much email beyond work, your ISP’s email, and Gmail or Yahoo!, then Apple’s own Mail app works OK.
Mail, though, does not sync accounts from device to device. But it’s free.
Spark runs on macOS and iOS, but also synchronizes accounts and settings between devices. That means what you set up on the Mac is easily setup on iPhone or iPad.
This one is free– for now– and the easiest to use email app for everything but spam.
Airmail works with SpamSieve so once you set it up on your Mac it will identify incoming spam, segregate it to a spam folder and that keeps spam off iPhone and iPad. Syncing accounts between devices is uttering painless.
Easily the most customizable email for Mac, iPhone, and iPad, but indeed, it’s an acquired taste.