IMAP feature is required when you want to read Gmail messages on external application. For example, if you want to access Gmail account within Outlook Express, IMAP should be enabled. This will allow download of Gmail account contents within Outlook Express interface.
Steps to Enable IMAP in Gmail
1. Login into your Gmail account.
2. Click Settings link at the top right part.
3. Then click “Forwarding and POP/IMAP” tab.
4. Click enable next to “Enable IMAP” under IMAP Access option.
5. Click Save Changes button and its all done.