One thing that I've enjoyed is how for decades Mail.app has been first and foremost a generic IMAP mail client, with features specific to iCloud Mail being far

One thing that I've enjoyed is how for decades Mail.app has been first and foremost a generic IMAP mail client, with features specific to iCloud Mail being far and few between. It works exactly as well with e.g. Fastmail as it does iCloud Mail. Conversely, iCloud Mail is just plain old IMAP and works fine with other generic IMAP clients. Compare this with Gmail for example, which has a crummy IMAP implementation, and though the Gmail mobile app supports adding non-Google IMAP accounts, it clearly prioritizes being a Gmail client.