If the sender of a message is not located in DayLite, DMI will give you the option to create a new contact in DayLite. When you click on the button, the create contact/organization screen will drop in DayLite with the firstname, lastname and email address filled in (for the Public Preview, you will have to remove then add one character in the firstname/lastname field then tab out to enable the 'create' button and trigger the duplicate checker).

If you find a duplicate contact, go that record and add the new email address that contact.
Once you've created (or updated) the contact in DayLite, go back to Mail and click on the Refresh button.