I use Thunderbird, which is the most full-featured of the three or so desktop email clients for Linux. Thunderbird aggregates my many email addresses in one place and allows me to manage my email mostly in the way I would like.
My biggest problem with Thunderbird is simply that it is not a joy to use. As a tool, it accomplishes the job. Maybe that's all I can expect.
It seems like many people are releasing very cool webmail solutions. But webmail is kind of gross. I have yet to find one that does a great job aggregating several accounts in a seamless way. It requires my webserver to be able to see the folder where I store my mail, which is a big concern (or alternately it requires me to run an intranet-only webserver for the explicit purpose of providing webmail, at which point why not just run a desktop client?). And I am extremely skeptical of any end-to-end encryption strategy using webmail...I understand that an HTTPS connection to the server + secure encryption on the server before transmitting should be sufficient, but it doesn't feel right. I'll be over here in my tinfoil hat if you need me.
Writing a desktop email client for Linux is not the way to get massive loads of HN karma. It's kind of unsexy, and since in the Year of our Lord 2013 it is still unnecessarily difficult to write something that looks nice and runs on multiple platforms, only a few die-hard throwbacks will be grateful (and half of them are running mutt or pine anyway and will sneer at your sissy X windowing system).
OK, so I probably answered the question I opened this comment with, but it was rhetorical anyway.
Speedwise its not up to par either I think, searching is quite bad, but this might be issues really with imap rather than anything else. My last efforts have been beefing up our mailserver and also adding full text indexing for searching which helped a bit but there is still a long way to go to get it really instant.
It gets the job done, but I've seen enough great software that I feel like it should be so much better.
But the truth is, email clients have always kind of sucked like this.
What are the alternatives?
mutt and pine (or alpine) running in a terminal are fairly cromulent alternatives to those who don't mind navigating with the keyboard and working modally, but that's not really how many of us prefer to work with our email.
(I have over ten years of email archives, and in order for me to switch, the new system will need to be able to import and search these archives.)
That's been in my head for years now.. just not sure how to have something accessible, with a free tier and make money.
I'd kill for a decent cross platform mail client. They're all shit. I've literally tried them all. Always end up back on mutt + procmail.
Scarily enough, a version of outlook express that bothers to respect standards, has a decent on disk format (maildir please) and a decent editor would hit the mark.
Are they (small) consultants / consultanties who are likely to install your system for other to use and either provide feedback for environments you don't / won't have access to; or provide patches.
Or are they companies like Google / IBM / Oracle. If the later you should always choose a copyleft licence like AGPL.
Well all those companies, when they have the chance to release Free Software pick BSD / MIT style licences; because all of them have the engineering resources to fork something and take it in-house and improve on it AND continue to pay the 'stupid tax'.
But if look at something like the Linux kernel, which is GPL'd, all of them have had to contribute back.
You should pick the licence that makes things easiest for you.
Each comes with its own set of communities; I know people who will not contribute to GPL projects, whereas others never contribute to BSD / MIT projects.
Plenty of organisations make money from GPL software (e.g. Redhat) and plenty do from MIT (but I can't think of any atm - late night, wine, etc.)
Summary: Pick what is right for you, and ignore nay-sayers. You will have customers - if the product is good - no matter the licence.
A veritable design plague.
Copyright very much applies to MIT/BSD/GPL licensed code as well. In fact, the mechanism of the GPL requiring changed+released versions to include an offer for the (modified) source code depends on copyright.
Edit: Technically speaking, it IS already on github, just not on a public repository :)
My entire workflow for keeping track of projects I want to follow centers around GitHub, so I can't wait :P