Cannot log onto 1 of my 4 email accounts

I have four e-mail addresses at xfinity.comcast.  I can get my mail (through Outlook) for three of them consistently.  One of them I consistantly have trouble with.  I can log onto xfinity.comcast.net with the same userid and password and get my mail, but it consistently has trouble with an e-mail client.  Thunderbird mail used to have the same problem with this one account.