Try these steps-

Go under Tools: Options: Viewing Mail. Try toggling the setting "Use Microsoft's
viewer". If it's checked, uncheck it and try printing. If that doesn't help, try
checking it and try printing..

Under Tools: Options: Viewing Mail, try unchecking "Show message preview pane". Then
try printing.

If it still doesn't work, try re-installing your print drivers. Check with the
manufacturer of your printer and make sure you're using the most current print
driver. Restart Windows, then try printing.

If you still have a printing problem try opening Microsoft WordPad or NotePad. Do
they print properly? If not, it points to a system-wide problem.

(From Eudora's Support Documentation - Document ID: 1484HQ)