Solution 1 :
This could be an Address Book issue. Sometimes people try to send email to people in their address book but have an old email address entered. Make sure the person sending you e-mail is entering the email address manually. If this fixes the problem, make sure the correct email address is entered in their address book.

Solution 2: Make sure that the incoming mail server is spelled correctly.

Solution 3: for Outlook Express:
1. Click on Tools.
2. Click on Accounts.
3. Highlight your mail account by clicking on it.
4. Click on Properties.
5. Click on the Advanced tab up on top.
6. If you have ‘Leave a copy of messages on server’ checked, make sure that ‘ Remove from server after # day(s)’ is checked. Otherwise the messages will fill up your mail account and eventually no one will be able to send you new mail.