I am no expert on this and have Eudora 5.2 not 4.3 but this should work I think. The HTML mail must be sent in PLAIN format not in STYLED format. This is because if you send in STYLED format the HTML characters such as < > will be displayed as characters and will not be interpreted as HTML mail.
Also, pictures, gif animations etc should be saved on a web site.
So the steps are as follows: -
1. Paste HTML code into new Eudora e-mail.
2. Change location of pictures etc (if any) to the web address.
3. QUEUE message, at CHECK SPELLING BOX click JUST QUEUE, then on SENDING STYLES click PLAIN.
When forwading HTML mail, RIGHT CLICK the incoming mail to be forwarded, then VIEW SOURCE, then copy & paste to new e-mail, and proceed as above.