I am having problems with my Eudora OS X version 6 with "replying" and "forwarding" email. I can send an email to myself and reply - but can not "reply" or "forward" other peoples emails. It has been working fine. It is the "paid mode". I had the same type of problems back in September 2002. Here was the Eudora tech support advice.
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> You very possibly have a corrupted Out mailbox - do the following:
Hold down Control+Shift+Tools\Options - click on Yes

This will allow you to reset your windows without loosing the POP and Smtp server information.

Eudora will shut down after you do this - when you open it again your default settings should be back. Before reopening Eudora do a search for out.mbx out.toc when found rename out.mbx -out2.mbx, out.toc -out2.toc Now reopen Eudora and you should be fine. <HR></BLOCKQUOTE>

QS 733, 896 MB RAM, OS 10.2.8 Here are 2 threads with info - I have tried the fix as described above no luck. It was for ver 5.x

This thread helped me last year. Maybe this one here too.