I use Thunderbird by Mozilla and have for years now. It is the only email program I recommend to my clients. Thunderbird isn’t without faults though with having some simple features missing the developers tend to leave a lot of functionality up to 3rd party plugin designers. Here is some of the most used plugins for Thunderbird that I couldn’t live without.
Contact Tabs enhances Thunderbird to give you quick access to all your contacts directly from Thunderbird’s main window: It opens a new tab with the contact details.
This extension is for those TB3 users who want the multiple-columns view that TB2 allowed.
This extension allows for customization of the folder pane. Accounts can be rearranged and the startup folder can be chosen. A must have when you’ve got more than 1 account, I have 30+!
Adds a summary of mail accounts and folders to replace the account central pane and the empty folder preview.
This addon provides a conversation view for Thunderbird, including contacts tooltips, attachment thumbnails, quick (inline) reply, integration with other addons, and much more. This is one of my favorites as I can see what I replied in previous emails without having to dig through my sent folder.