OK, fair enough, depends who provides the app. So far the project author(s) seem anonymous.
(And no, the fact of being open source is not enough, at least not until certain popularity is reached)
I've tried many XMPP clients on mobile but I would be afraid to test many e-mail clients.
We are http://www.open-xchange.com, http://www.dovecot.org plus people from the Delta Chat community and http://spikenow.com - plus whoever whats to join.
People generally take issue with handing over access to their email account to third party services. People don't generally take issue with a client under your control accessing an account under your control.