base64 - a picture? (that would display in alpha=numeric in plain text mode, I think, but I'm not sure)
Have you disabled your Internet Explorer HTML email viewing in Eudora?
If so then you may need to re-enable it to read/view the html email, so you need to Check/tick the Use Microsoft's viewer.
select Tools, Options from the Eudora menu.
Go to the Viewing Mail category.
Make sure Use Microsoft's viewer is checked.
Worth a try anyway.