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

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

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.