Open the installation folder of Eudora and locate the .toc file which corresponds to the section your email is. For example, if you email is in Archived directory, locate archived .toc file. Make a copy of that file and save it somewhere else then delete the original file from the Eudora directory and start the application. It will tell you that the table of contents needs to be created. Allow this and wait for the process to finish. In many cases this is the solution, but if the attachment is badly corrupted then I am afraid it is lost forever.