In an interest to move my personal data away from the hands of corporations, I am looking to close my gmail account and start using a self-hosted solution; however, I currently use the web front-end client for gmail a lot.
I already have my own server and domain. I am flexible about the mail server-side software, and more interested in finding a full-feautred open source replacement for the gmail web front-end client -- but if someone recommends something that provides both the backend and the frontend in one package, I would happily use that.
- Obviously, it must be hosted on my personal server and can't upload my personal info to anywhere.
- Open-Source (although I don't necessarily need it to be free as in freedom), otherwise I would have no way to be sure that  remains true.
- Threaded conversations
- "Tagging" or moving conversations to certain "Boxes"/"Folders" (tagging is preferred, so I can have a given conversation in more than one category)
- Basic filtering functionality for incoming messages
- Basic Address Book functionality (be able to search for colleagues by name when composing a new message, provided I've added them before)
Desired (but not required) Features
- Easy-to-use and nice-enough-looking interface
- A "markdown mode" when composing messages
- Spam-detection engine
- Support for something similar to gmail's "plus addresses" - e.g., email sent to firstname.lastname@example.org will be seen in my email@example.com inbox. I think this would only work if it was supported by the back-end as well.
- A calendar feature (if that calendar supports automated notifications, even better -- and if it supports SMS notifications, I'll be blown away), but I can always install a separate calendar application. Some sort of open-ended integration/plugin system with other calendar softwares would also be a great option.
- A moderately active development community that is open to source code contributions.
I'm open to software written in any language. It doesn't necessarily need to be free (as in coffee), but that does usually come with open-source.