I want to send an email to a large group (>50) and have created a smart group for this. Using the scheduled mailing function I selected this group as the recipients but now an extra box appears with an unsubscribe group, what is this for and can I do the mailing without this?
The unsubscribe group provides a way for recipients to opt out of future mailings. It isn't possible to send a mailing without this.
Please read the documentation for more information about how this works: Mass mailings using CiviMail
Here's an extract that provides an overview:
For mailings to search results, you are required to choose a Group from the Unsubscription Group dropdown menu. Here's why: Every mass mailing needs a way to track unsubscribe requests. A simple "unsubscribe" process is required by law in many countries and may help prevent your mailings from being treated as spam. Mailings sent to Groups have this capacity built-in. The next time a mass email is sent to that Group, anyone who has unsubscribed will not be included. However, mailings sent to search results do not have this built-in way to track who has unsubscribed, so you need to provide one.