Has anyone been able to change this box to be a multi-select list?

standard email fields only allow one address as far as i'm aware. we've created custom fields to have multiple emails and then developed a process to send all at once.

You can enter multiple email addresses by separating them with a semicolon. It is important to separate the addresses with only a semicolon and do not include any spaces.

We are using a solution created by Luxent to be able to send emails to muliple contacts and based on the type of notifications they want to receive. In sure if you have a developer in house, they should be able to do the same for you. You need a script to gather the email addresses of all contacts, and then a workflow to send the email.