Hi curiousgeorge - I wanted to jump in here and let you know that you can send to only a portion of the contact list by creating a mailing list sample. Here is the Creating Mailing List Samples support page, which will walk you through to how create a sample so that your email is only sent to a subset of the contact list rather than the entire list! If you have any follow up questions on how to set this up, you'll want to reach out to our wonderful Support team, as they are best equipped to help!