I did something similar for a site a while ago, using Rules in Entourage (though it would also work in outlook express.
Presumably all the submitted email addresses arrive in your inbox as emails with a subject like 'subscribe' or similar. Set up a group in your entourage address book with the name 'Website subscribers' or whatever you like.
Now set up a Rule that if 'subject' is 'subscribe' then add sender to mail group 'Website subscribers'. Obviously this will only work when new messages arrive in your inbox. However you can simply apply the rule through the menu and it will add those addresses already in your inbox. You could also set up a rule to remove addresses if they send an email with the subject 'unsubscribe'. Not sure if that's exactly the right settings, as I'm recalling this from memory, but you get the general idea!
All you need to do now is write your newsletter and put the website subscribers group in the 'To:' box! This also means that none of the recipients of the newsletter see anyone else's email address.
Hope this helps!
