If you ever try to write a email client, you will immediately realize how difficult, if not impossible, to fix all the bugs for a email client. It is a multi-different-protocol-version-async-client-handling-same-database-with-thousands-of-race-condition backwards compatibility nightmare.

Writing a email client with support of just up-to-date protocols and assume it is the single client that will operate that account is trivial, write one that covers all corner cases is a totally different story.

Adding a mobile version, MS Exchange integration, supporting OAuth2 login, refreshing the UI periodically because otherwise everyone whines about how "dated" it looks.

Their RSS could use some improvements, but the same could be said about all of them.