This is the software publisher's description.

Eudora is a mail and news application based on the open source Thunderbird client from Mozilla. It's not our intention to compete with Thunderbird; rather, we want to complement it.

We are committed to both preserving the Eudora user experience and to maintaining maximum compatibility, for both developers and users, with Thunderbird. It is our goal to build a single development community around Thunderbird and Eudora, so that both mailers advance faster than they previously have.

Whereas "Eudora" is a branded version of Thunderbird with some extra features added by the Eudora developers, "Penelope" is an extension (also called an " add-on") that can be used with either Eudora or Thunderbird.

The Eudora installer includes the corresponding version of Penelope along with it so there is no need to install Penelope if you are installing Eudora. Most features in Penelope can be accessed when used with Thunderbird, but there are a few that require Eudora in order to work correctly and it's not something that gets tested.

Sometimes in documentation there is a need to differentiate the older versions of Eudora made by Qualcomm from current Thunderbird-based versions of Eudora. This will normally be done by labelling the older versions of Eudora as "original Eudora" or "Classic Eudora".