Open Eudora.  From the Tools menu, select "Personalities".

The pane on the left side of the window will show your list of accounts (which Eudora calls "Personalities").  Your Hunter account may be the Dominant personality, or one of the ones under it.  Whichever personality represents your Hunter account, right-click on it and select "Properties".

In the Account Settings box, check the box marked "Use submission port (587)", and choose "If available, STARTTLS" in the pull-down menu under "Secure Sockets when Sending".

Click OK to save the settings.  You are now correctly configured for Hunter email, and if you were trying to send mail from a home DSL or cable modem connection which was not permitting you to do so, this will probably resolve your issue.

Note that, on your first attempt to send email, you may receive an error message about the security certificate being used by the mail server.  This is due to Eudora's status as a program no longer under development; it does not recognize some recent certificates.  Although we do not like advising users to ignore security warnings, if you wish to use Eudora, you will need to accept the certificate in spite of the warning.