We use the program Mozilla Thundebird for doing all our email. Its pretty easy to use, reliable and does what we want. One of the best things about it is that people around the world have written lots of plugins for it to make it that little bit better. Here are some of my favorites:

• Enigmail gives you an easy way to encrypt outgoing decrypt incoming messages. You need GnuPG or PGP to use it.
• Bork Bork Bork! changes your messages to read like the Swedish Chef from The Muppets :-)
• CompactFolder adds a toolbar button to compact the open mailbox. Very useful.
• Contacts Sidebar is something I wish Thunderbird had built in, but now I can have it anyway. It shows your addressbook under the mailboxes sidebar for easy access. Simply wonderful!
• Easy Get Mail Button adds a "Get Mail" button to your toolbar that only fetches mail for the account you've selected - handle if you've got oodles of accounts (like we do).
• Display Mailing List Header adds an extra row to the message header with links to subscribing / unsubscribing from email mailing lists.
• Leet Key is kinda like Bork Bork Bork only lets you convert text (either what you write or read) to l337 5p3@k, morse code, etc.
• MailTagger adds smileys to your messages, what more do I have to say? :)
• Move Search Items moves the Find and Search fields into the toolbar or titlebar to save screen space.
• Quote Colors lets you change the colors displayed for different levels of quoting (i.e. for replies to replies).
• Text Size Toolbar adds a very simple feature missing from Thunderbird - you can now easily select the text size !
• Timestamp simply adds the date & time to a message you are writing.
• View Headers Toggle Button lets you easily select between showing the summary headers or all headers.

Here are some others that look good:

• AddressContext adds extra menu items to context menus. I haven't tried this one yet.
• Attachment Extractor simplifies saving out attachments from multiple messages at once. Could be useful.
• MessageFaces lets you view avatars that some people add to their emails.