Maybe the DNS zone has not been cached for you yet by your ISP. I can reach your A record for your domain at DreamHost and can also see your MX records point to DreamHost.
Give it some time (0-73 hrs) and try again at https://webmail.example.com/
Try flushing your local DNS.
Opinions are my own views, not DreamHosts’.
I am NOT a DreamHost employee OK! :@
Act on my advice at your own risk!