Thank you for your post.
I have not encountered this error, and I don't see any other customers reporting this issue. I know that doesn't help you, so let me find out some additional information so I can try to duplicate the problem here.
To what server is FileMaker Server sending the email? That is, what mail server is Eudora pulling the messages from?
What version of Eudora are you running?
Are you removing the messages from the server after you download them to Eudora? If not, are you able to access the messages from another email client?
Try sending the messages to two different email addresses, and access each email address from a different mail client? If unavailable, try accessing the messages from another machine.
Any other information you can provide may be helpful in determining possible causes.