I quite disagree here. Servers and clients are very different in their architecture and implementation, and you are going to have very little opportunity for code reuse or even knowledge sharing between the people implementing the client and those implementing the server [1]. I don't think you can turn email servers into a profitable revenue stream unless you do full SaaS, and I don't think Mozilla is really capable of putting in the resources to make a Mozilla Email SaaS feasible.
A better bet would be to try to put together something to more effectively cobble together the existing open-source email server components and do the installation and support as a service, but I again don't think that Mozilla is the organization best-placed to provide that service.
[1] The big exception here is LDAP. But Thunderbird doesn't even really maintain its own LDAP library--it should move to WinLdap/OpenLDAP depending on OS.
True, but also the only way to really know how to develop a good one is to have experience with building the other.
Ximian's Evolution?
I would love for someone to do this, so we could have a nice, self-hosted store for our vital data but still access that data from all our devices, over VPN, etc.
The problem, as always with these things, is who that someone would be. While many HN readers might appreciate the potential advantages, I fear the target market for such a product is significantly smaller than for Thunderbird, which for better or worse is itself aiming at a relatively small minority of users in the era of Google Mail and friends.
For anything it's worth, I'd contribute financially to such a project personally and so would my businesses, since it would be a significant benefit in each case.
o365/exchange, gmail/gdocs/gcalendar and fastmail are probably the big three providers that should have seamless integration at this point. A SaaS option from Moz would be nice, as would a self-hosted option. I bring these up in line with Moz's efforts to get VPN subscribers, they could likewise do groupware.
Why's Mozilla got a stock ticker if it isn't publicly traded?
Second to this, they should probably work on their own first class mail and calendar server as at least open core, and probably as SaaS as well. As much as I like some of the mail hosts out there, for some, the ability to self-host is big. The various pieces and software you have to cobble together are cumbersome, and even then, you don't get anything close to what Exchange+Outlook gives you.
It would be nice to see Thunderbird fill this space, it used to be what I would consider a best of breed email & newsgroup client, now, I'm not sure I would say anything close to that.