I've set up countless (PHP/MySQL) mailing lists in my day for lowish-paying clients, and have always just had the results outputted as a comma-separated list of addresses for them to copy and paste into the Bcc of their 3rd party email (gmail, hotmail, etc...)
As clients become more sophisticated, I'm wondering if it's possible, using PHP, to deliver addresses somehow automatically to their contacts list stored on the 3rd party email account. Do any email servers allow you to interface with them like this, through your scripts? I know they facilitate the bulk import of address books, but is it possible to have server-side scripts hook up to their system and deliver addresses, dynamically, into an account? What information would I need from them? This seems like a long shot, but just wondering.
In the past, with a small amount of recipients and low-security email situations, I've used the mail() function to set up a custom mail-sending interface that utilizes their addresses stored in the MySQL database I've set up, and allows them to skip over the "copy and paste" step, but am wary of using this on a large scale.