First, make sure you're adding email addresses in the People section, not at company level, and that you have Use for Online Enquiries ticked.
Secondly, if the contact has already been added to the sendout, even if it was then removed, it will be 'cached' and so not get any new information from the database until you tell it to by clicking the Get New Addresses From Database button.
New emails not showing in Online Enquiries
I can't use the email addresses we just added to the contacts database
Written by Norbert Botocan