Apple Mail access to Comcast Business EMail

On our campus, we have MacBook Pro, iMac, and MacBook Air computers.  I have been unable to setup access to our comcast business email using Apple Mail v.8.2 OS Yosemite 10.10.2  I've used the IMAP and SMTP servers listed in the DNS settings --imap.ourname.comcastbiz.net  and  smtp.ourname.comcastbiz.net  as well as   imap.dnsname.comcast.net  and  smtp.dnsname.comcast.net  neither pair works.  I get errors that ask for password even though I have entered it correctly --repeatedly-- and I know it works since I can log in to the portal and check email just fine online. C Learly I am doing something incorrectly. But what? Any ideas?