Eudora is a mail and news application from Qualcomm based on the open source
Thunderbird client from Mozilla. Eudora is extended with "Penelope add-on" which also works for Thunderbird, but the Eudora installer includes some other options with this extension, which are not available for Thunderbird. Don't worry, there is no need to install Penelope if you installed Eudora, as it comes bundled with it. Being based on Thunderbird, it works on Windows, Mac OSX. Starting with version 8, Eudora it also works on Linux.