Most of our servers have a 60 checks per hour limit. If you hit this limit in any hour you’ll be locked out for the rest of the hour. You will need to click Tools > Options > General > and disable where it says “check for new messages every xx minutes”.
If you do not want to disable it, just set it to check less often. Keep in mind the more computers you have auto checking or checking the mail, the quicker you will hit this limit.