CCG jokes aside for a moment - this is actually a pretty good mail client.
All the usual features are there - including a learning spam-filter that gets progressively better.
It has some missing features ('get mail from all my accounts' being the most glaring) - but these can be remedied by plugins.
It doesn't merge all accounts into one inbox. But it does have powerful message filtering rules.
It does have the ability to leave messages on the server, and only download them if your rules allow it or you click on them - but this feature is currently dangerously broken. The ReadMe says to not risk it.
Unlike FireBird?, it's not unusably slow. The option to turn off HTML email is fairly well hidden, but it is there. (Add domain '*' to your defaults list) Sadly, it default to sending HTML. Change that before use.
Fast, effective, correctly imports your existing mail folders from lookout... I heartily recommend this. Its only problem is that the spam filter is so good, my 'give me mail' button is no longer functional.
(Opera's mail client is becoming unusably crashy for me.)
Currently up to [Version nine] (that link should always go to the latest one) and has a wonderful new feature - built in RSS.
I use ThunderBird because I know no self-respecting geek ought to use Outlook, but TBH there are so many irritating usability glitches in ThunderBird that I'm tempted to go back to Outlook or OutlookExpress?. For example:
If you do Reply All to an email which was sent to you and others, it puts you in the CC field for the reply, and you have to remember to manually remove yourself, otherwise you get a pointless copy of the mail which is already in your Sent folder.
In Outlook you can drag files from any explorer-type window onto an email you're composing, in order to attach them. In ThunderBird, you can't - you have to use the Attach option in ThunderBird's menu system, and navigate through your filesystem using ThunderBird's own file-browser dialog rather than using the window where you've already found the file(s) you want.
Maybe I just have a different version of Thunderbird, but this works for me - although the files have to be dragged into the area with the email addresses etc rather than the body. --Edwin
For saving incoming attachments to your filesystem, you can drag and drop, but you can't multi-select using Shift. You can't multi-select using Ctrl either, although it looks like you can, because it highlights multiple files, but in fact only one of them is selected, as you see when you try to drag them all to a filesystem location. If you use the ThunderBird menu options to save attachments, you can save them one at a time or Save All, not a subset of them. If you do Save All and they're overwriting existing files, you have to confirm each one individually - there's no Yes to All button.