I got it to work. The way it needed to be done was webmail.yourhost.co - DUH!!!
It is not either of the three options you stated (Neomail, Horde, or SquirrelMail) it is something different (uebimiau???).
It works.
Does anybody know how to change it. I believe I do not have CPanel available. I have the special shared plan (10 domains included) and I have a helm panel but no CPanel. If this is incorrect can somebody offer advise.
Thanks in advance.