When are we going to have Activesync support in K9?
https://github.com/k9mail/k-9/wiki/K9MailAndExchange
K-9 uses WebDAV to communicate with Exchange. As of Exchange 2010, WebDAV is no longer supported. For this reason, K-9 is unable to work with Exchange 2010 at this time.
http://codethis.wordpress.com/2011/09/29/building-k-9-mail-with-exchange-activesync-support/
K-9 communicates with Exchange through and outdated and problematic protocol: WebDAV. WebDAV support is turned off by default in Exchange 2007, and completely removed in Exchange 2010. This of course implies that K-9 does not support Exchange 2010.
Exchange ActiveSync Development
http://blogs.msdn.com/b/webdav_101/archive/2011/09/29/new-to-exchange-activesync-development.aspx