Make sure you set it up exactly by looking at Flash / Non Flash
Make sure your user name and password is all lowercase.
Make sure you have created the email address in your control panel.
Still not working?
Change your incoming and outgoing mail servers from mail.yourdomain.com to the ip address listed in your welcome email.
Make sure you have "My server requires authentication" enabled otherwise it will not work.
Are you able to receive emails but not send?
If you can receive mail but aren't able to send this means one thing.....
Your internet provider is blocking you from using anyone's outgoing mail servers but their own. Many major ISPs are blocking outgoing emails. The reason they do this is to prevent you from using their internet connection for spamming purposes. You will still be able to send emails, but only by using your ISP's outgoing mail server. Every ISP's outgoing mail address is different, but for the most part they will be mail.yourispname.com. For example, if you had hosting with AOL.com, their outgoing mail server would look like this: mail.aol.com. If mail.yourisp.com does not work and you are still unable to send emails, you will need to call your internet service provider to find out what their outgoing mail server is.
We also have exim running on port 26 for this very reason. A possible work around is to change your outgoing mail to port 26.
Are you able to send emails, but not receive?
Either your domain is out of disk space, or your e-mail address itself is out. You will need to increase the amount of disk space to receive mail again.
Does it say "please enter your username and password for the following server" and you are sure you're using the correct information?
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.