IMAP (Internet Message Access Protocol) lets you download messages from gmail so you can access it with a mail client like Outlook Express or Apple Mail. IMAP syncs with gmail. For example, if you read a message in your mail client, it will be marked as read in your gmail.
Standard IMAP configuration
Please note that if your client does not support SMTP3 authentications, you won't be able to send mail through your mail client using your gmail address.
If you're having trouble sending mail and encryption is on for SMTP for your mail client, configure your SMTP server on port 465 or 587.