My office uses Macs, but most of our clients use Windows and Outlook. Whenever people send invitations from Apple Mail to a Windows/Outlook machine, the invitations are garbled and look nothing like the nicely formatted invitations that Outlook people are used to. We also have no tools to view shared calendars, so we can choose mutually open time slots, and other useful calendar features that Outlook has and Apple Mail seems not to.

So is there a plugin or third party program that will give Apple Mail the nice calendar features of Outlook?

(By the way, I've looked into actually buying Outlook for Mac, and the pricing is kind of prohibitive, because you MUST buy the whole Office Suite, which we already have, there's no upgrade path, and there's no volume discounting.)