If the account is a POP account or the settings in solution #5415 do not work, try cleaning junk files and/or doing and registry cleanup with CCleaner or another comparable program.
- Link to free download of CCleaner: http://download.cnet.com/ccleaner/
If you're an Exchange user using Outlook, be sure that Outlook is in Online mode. The easiest way to check for this is to click on the File menu. If you see a checkbox next to 'Work Offline', then Outlook is not connected to the server. Simply click on 'Work Offline' to remove the checkbox, which should put Outlook back online. Once online, you may want to click the Send/Receive button to send any messages left in the Outbox.