I somehow haven’t gone into detail on Thunderbird before, so here we are. If you aren’t aware, Thunderbird is a FOSS mail client and calendar (with support for messaging via IRC & co., but who would use it for that?).
Perks
Native Application
Since this is a program that is stored and runs directly on your computer, you don’t need to download the whole thing every time. This allows it to be much more feature-rich than your typical web mail client, as I shall discuss. It even allows you to read (and write) mail without a persistent internet connection, since it can download your inbox (even with IMAP, with no configuration) and queue up messages to be sent once you do get a connection.
Multiple Addresses
The single biggest advantage of Thunderbird (or any semi-competent mail client) is that it can keep you logged into multiple e-mail addresses, at once. Firstly, this ensures you have a uniform interface for everything, rather than being at the mercy of whatever Google or Microsoft decides they want things to look like.
This has one particularly useful benefit for people with multiple accounts under the same provider. I, for example, have a personal address and a university one which are both served by Microsoft. To switch between them when using web mail, I have to log out and then log back in with the other one. In Thunderbird, they’re just both there, and I can switch between them instantly.
Keyboard Bindings
Thunderbird lets you do most daily tasks with the keyboard. I normally associate keyboard bindings with terminal-based programs which require the user to re-implement most of the functionality for themself, so this was a nice surprise. A lot of this is done using menu accelerators. To access the “eBay” folder in my “GMail” inbox, I can press the following: Alt-G o g i e
Flexible UI
I use a tiling window manager, so the size I allocate to a given window can change frequently. While a web mail client like Microsoft’s will struggle with less than the whole screen, Thunderbird handles it fine.
If it has the whole screen, display the message pane side-by-side with the mail list (View → Layout → Vertical). If it has ½ the screen, display it below the messages (View → Layout → Wide or Classic). Even when it only has ¼ of the screen, it can manage by simply hiding the message pane (F8).
Panes can also be resized by dragging the border, updating in real time (unlike Outlook webmail), and with the option to drag all the way across to hide an element completely.
Mail and Folder Manipulation
Thunderbird provides some fairly powerful mail filtering and search tools (which work quickly since the messages are downloaded) which make it easy to find a message among hundreds. I especially like the quick filter (Ctrl-Shift-k) because it keeps you in the same folder and provides incremental results (provided you stop typing for a moment).
Challenges
Sometimes it won’t automatically get the settings right for certain mail providers, notably Microsoft Exchange. In the cases of my school and university addresses, the issue was multi-factor authentication not being enabled.
There are also some services, such as Proton Mail and Tutanota, which deliberately make it hard to use a client other than their official ones, usually in the name of security. If there isn’t a solution, that’s the fault of the provider, not Thunderbird.
Lastly, Microsoft Exchange specifically pre-filters mail into a “special” junk folder that can only be seen through their web mail client. Once again, Thunderbird is not at fault here.
Mobile
There are apparently plans in motion to turn K-9 Mail into Thunderbird on Android. I’m currently using the GMail app on my phone, so this is something I’ll want to try out. However, I don’t think a mobile phone is good for anything more than reading e-mails and displaying push notifications (which it’s often more convenient to not have when concentrating on your work).