I have written (or fixed) a few plugins for Eudora (for Windows) and made them available for download below. I have tested the plugins with Eudora 4.3.x, 5.0.x and 5.1.x (and in addition they should work with 4.0.x, 4.1.x and 4.2.x), but because they are distributed free of charge, there is no warranty provided:
The plugins are provided "as is" without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose. The entire risk as to the quality and performance of the plugins is with you. Should they prove defective, you assume the cost of all necessary servicing, repair or correction.
In no event will I be liable to you for damages, including any general, special, incidental or consequential damages arising out of the use or inability to use the plugins (including but not limited to loss of data or data being rendered inaccurate or losses sustained by you or third parties or a failure of the plugins to operate with any other programs), even if I have been advised of the possibility of such damages.
The plugin may perhaps not work as intended if the entire text is in uppercase (e.g., it will turn 'Ů' into 'Th' without checking if the surrounding characters are also in uppercase), but you shouldn't be shouting in your e-mail anyway :-) or you can use the UprLwr32 plugin above to fix it.