Acho que o DavMail deve ser capaz de resolver o seu problema. Para citar a página
Ever wanted to get rid of Outlook? DavMail is a POP/IMAP/SMTP/Caldav/LDAP exchange gateway allowing users to use any mail/calendar client (e.g. Thunderbird with Lightning or Apple iCal) with an Exchange server, even from the internet or behind a firewall through Outlook Web Access.
Ele fornecerá back-ends POP / IMAP / SMTP / CalDav / LDAP para um gateway do Exchange OWA.