Most clients now special-case Gmail because they have all but deprecated POP and IMAP.
Push notifications, essential for seeing mail "in time", are a proprietary, inconsistent mess.
Receiving servers use all kinds of voodoo to determine whether a message is acceptable to them, much of which is powered by proprietary software and semi-secret block/allowlists.
Attachments are barely usable because Outlook uses a format that many other clients don't understand.
Large providers have hardcoded logic for marking messages from "trusted" senders like banks and online shops, so some senders are more equal than others.
Meanwhile zero meaningful progress in decades to address the protocols' basic shortcomings like lack of encryption, replies that don't involve sending hundreds of pages back and forth, ...