Micah uses Thunderbird with Pop 3 protocol. Leo says that most email servers have moved to IMAP now, keeping your email on their servers. Pop allowed you to delete from the server. But more people wanted access to their mail in the cloud than just on their computer, so IMAP was born. If you want to delete your email off the server, you can manually do that. But that's why Leo recommends Proton Mail. It's end to end encryption so no one can read it.