When user login at webmail.domain.com and select Horde as webmail interface, a strange port get appended to URL 8443. Horde does not respond on that URL
This would have to have been set up by someone - webmail uses 2096. You might check the redirections for the user's account as well as ensure that webmail.domain.com resolves to the server.
If webmail.domain.tld goes to port 8443 it's incorrect though. This isn't something we would ever set up. Webmail, including horde listens on 2095/2096