While the classic Mac and Windows versions of Eudora shared the same design aesthetic, they had entirely separate code bases. Eudora OSE is a multi-platform application based on a single, unified code base, and offers variants of both classic interfaces to users on any operating system.
The 3-Pane (MDI-like) style will be most familiar to Windows Eudora users. This is essentially the same as the standard Thunderbird interface, with a 'docked' mailbox list along the left side, and a 2-pane message list and message preview on the right side.
The 2-Pane (Mac-like) interface causes mailboxes to open in a similar manner to Mac Eudora; 2-pane message list and message preview. The list of mailboxes will open in a separate window.
The latter interface will likely be bothersome for Windows users as Penelope does not support the same MDI interface used by classic Windows Eudora. As a result, each of the open windows is listed individually on the Task Bar. Mac users will find this interface to be more like Mac Eudora.