Mailpile is fundraising, and I think it is worth supporting.
The obvious reason is that we need better self-hosted mail options. Nearly every person I know that switched to GMail found it welcome after “that horrible squirrel mail”. SquirrelMail is fine software, but it lacks the front-end design that folks expect. Mailpile could fill that gap.
The less obvious, but more important, reason that I want to support Mailpile is to see how they implement encryption. There are already plenty of issues in their queue where they are hashing it out. I’ve said this since Diaspora started up, having private communications in a federated social network is useless unless you ensure something akin to public-key encryption, where the private key is never given to the site operator. I’ve proposed in the past that we just flag messages as encrypted so they don’t show up in public streams, and then use a browser plugin to decrypt the message, but that is based on my low-level understanding of what is possible, and I hope that Mailpile does something more elegant.
So check it out, and consider supporting them. 
Oh yeah, one more thing: this project doesn’t approach the difficult area around self-hosting email. Malicious traffic and misconfigured services have made it so it is difficult to get one’s own mail to deliver properly, without the resources of a dedicated email team to field blocks and surges from the federated mail network. On the other hand, if we had a nice package that folks wanted to use, maybe we could start fixing that area as well.