Well, a DNS error is typically local, please try fingering your DNS server...or if you give me your domain, I can finger it over here and make sure you have a CNAME or A record set correctly.
Originally posted by _spawn well am getting to the index file in the webmail directory; it is when it metarefreshes to the /horde directory is when it gives the dns error.
I had the exact same problem. I tried stopping and starting the related services, but ended up having to reboot the box. Everything worked after that. Very strange....