Do you use gmail in your applications as well? I find that using gmail as a mail provider for my software, you are frequently notified that you haven't physically logged in in a while and that you need to do that before you are allowed to send your email. Also, if you're living in one country and your hosting is in another country, gmail might block your mails form being sent. What's your experience with gmail?
Rainloop and Roundcube looks interesting, are those just mail clients or do they offer servers as well?