IMAP is a mail access protocol that allows you to access and manage your mail from the server without downloading the mail.
To setup a IMAP mail access for Microsoft Outlook Express 5, here's a step-by-step guide:
Run Microsoft Outlook Express 5
Click Tools, select Accounts...

Click Add, select Mail...

Enter your display name (friendly name):

Click Next
Enter your email address:

Click Next >
Select IMAP as your mail server type and key in the incoming and outgoing mail server settings (eg. mail.geromail.com and smtp.geromail.com).

Click Next >
Enter your login name (full email address) and password:

Click Next >
Click Finish

Click Close
You will be prompted "Would you like to download folders from the mail server you added?"
Click Yes.
Click on the newly created mail account and select the folders to synchronize:

Click on Synchronize Account to update.
End of guide.