Mozilla Thunderbird is an email, newsgroup, and news feed client designed around simplicity and full-featuredness while avoiding bloat. Due to its modification with several patches, it is known on Arch Linux as Shredder. It supports POP, IMAP, SMTP, S/MIME, and OpenPGP encryption (through the Enigmail plugin). Similarly to Firefox, it has a wide variety of plugins and addons available for download that add more features.
To install, simply run:
$ pacman -S thunderbird
Encryption with EnigMail
The first step to setting up email encryption is to download the GNU Privacy Guard, or GnuPG. Several packages are needed for this. First are the 2 GnuPG packages:
$ pacman -S gnupg gnupg2
The GNU Portable Threads package needs to be download as well:
$ pacman -S pth
Once those are downloaded and installed, a C wrapper for GnuPG needs to be installed as well:
$ pacman -S gpgme
Everything else should already be part of your Arch Linux install, but you can install any potential unfulfilled dependencies.
EnigMail can be downloaded from here, at Mozilla's addon database. Make sure that it is installed as a Firefox addon and isn't automatically interpreted to be a Firefox addon. This can be done by going to Tools -> Add-ons, and clicking Install, then selecting the XPI addon package. Congratulations, EnigMail is now installed. You should now restart Thunderbird.
Creating a Keypair
A keypair can be created by opening Thunderbird and finding the 'OpenPGP' menu and clicking 'Setup Wizard'. The wizard will now help you create your keypair. All of the options are fairly self-explanatory and are not discussed in this article.
Sharing your Public Key
There are a variety of ways to distribute your public key. One way is to upload it to a public keyserver network. Another is to share it with friends who are also using email encryption.
Encrypting your Emails
First of all, encryption does not always work properly with emails containing HTML. It is best to make all of the encrypted emails you wish to send plaintext. This can be ensured by going to Options in the 'New Email' window and finding Format, then clicking on "Plain-text only".
Once you have chosen a recipient and title and have written your message, you can sign and encrypt the message by using the OpenPGP menu. Once that is done, simply click "Send" and your encrypted email has been sent.
This article will not go into the details of key signing.
Assuming that the email was encrypted properly, just trying to open it should result in a popup window asking you to type in your keyphrase. Do so now, and the email will be decrypted for your viewing pleasure.
Adding a Calendar with Lightning
Lightning is a plugin that brings Sunbird's functionality to Thunderbird. This integrates calendar functions into Thunderbird.
Adding a Webmail Account with WebMail
This issue is covered in the article Webmail with Thunderbird.