Welcome to Android Central! Which email app are you using, the stock Email app, the Gmail app, or some other 3rd party app?
If you're using the stock Email app, you need to make sure you've entered all of the account information correctly. Open the Email app, select the account in questions, tap Menu, then Settings, then Account Settings. Make sure all of those fields are entered correctly. Your email provider should have a support website that can tell your the name of the IMAP server, the SSL setting, and the port.