Thunderbird is a powerful free Email- and USENET newsgroup software package. Its source code is available for free, too.
The most interesting parts so far are under "Advanced" (The Options Dialog) and under "Background".
Items for which there is no link are not yet available, but planned.
General - Information
What or who is Mozilla? [External Link]
What is Mozilla Thunderbird, and why would I want it?
General - Information
Obtain Mozilla Thunderbird [External Link]
Screencast – Software – Thunderbird
Installing Mozilla Thunderbird
Flash, Audio, Screencast
Screencast – Software – Thunderbird
Adding an EMail Account
Flash, Audio, Screencast
Screencast – Software – Thunderbird
Adding an Outgoing Email Server
Flash, Audio, Screencast
Screencast – Software – Thunderbird
Adding a News Account
Flash, Audio, Screencast
Screencast – Software – Thunderbird
Adding an RSS Feed
Flash, Audio, Screencast
Screencast – Software – Thunderbird
Adding a Signature to your Messages
Flash, Audio, Screencast
Reading Emails
Writing Emails
Writing sophisticated HTML Emails
Folders
Filters
Tags
Handling Spam
Screencast – Software – Thunderbird
The Thunderbird Options Dialog
Flash, Audio
Screencast – Software – Thunderbird
Managing Sender-Identities
Flash, Audio, Screencast
Add-ons/Extensions
Themes
Backup of Profiles and Emails
IT/Computer – Internet – Email
About Email
Flash, Audio
IT/Computer – Internet – EMail
About USENET Newsgroups
Flash, Audio
IT/Computer – Internet – Email
About RSS Feeds
Flash, Audio
IT/Computer – Internet – Email
How Emails are transported (SMTP)
Flash, AudioEncryption
Encoding
Unsolicited Email / Spam
Email headers