In general we recommend our clients implement Exchange ActiveSync (EAS) when using Outlook for domains, however there are some limitations to this because Mac Mail and the Mac edition of Mozilla Thunderbird do not support it. However Microsoft have finally announced support for IMAP.
IMAP also allowed 3rd party developer integration, and multiple services have already announced compatibility with Outlook including TripIt, Sift, Slice, motley*bunch, and OtherInbox
