problem creating emails

Since yesterday I have tried to create some new email accounts and they don't work. I can see them on the web panel, I can login into the mailboxes account to change passwords and config it, but the account can't login on squirrel. Messages sent to those accounts bounce back. I have no idea what else to do, and precious time is being lost as these accounts were transfered from another host. Do someone have any idea of what could be happening ?

Is it a new domain?

http://WebsiteHacks.com
website development and computer help

One of the domains, yes, altough some of the accounts we created at first worked fine. Another two domain have been hosted for over a year. We are talking about 6 accounts in 4 different domains