Make sure that your email program is set up to use the correct servers to obtain mail from. When we talk in mail terms, you use two types of servers:
POP3 Servers (for checking/retrieving mail)
SMTP Servers (for sending mail)
For example, if you have the domain name registered with us, and you have hosting with us, make sure that you are pointing your mail program at pop3.yourdomain.com for pop3, and smtp.yourdomain.com for smtp.
If you use Outlook Express, you can find the options for checking where the server names are in the Tools menu, under Accounts > Mail > Properties > Servers.