The difference in code bases is striking [0]:

> The Windows version of Eudora is written in C++. The source tree consists of 8,651 files in 565 folders, taking up 458 MB. There are both production (“Eudora71”) and test (“Sandbox”) versions of the code.

> The Macintosh version of Eudora is an entirely different code base and is written in C. The source tree consists of 1,433 files in 47 folders, taking up 69.9 MB.

[0]: http://www.computerhistory.org/_static/atchm/the-eudora-emai...