If I include ?utm values in the link to our Subscribe Form, is there a way to track the Referral source of new newsletter subscribers?
I want to find out what the best sources of new subscribers is.
This is a great question :) You can add hidden fields to your subscription form, so that data is added to custom fields in your list without the subscriber seeing. To do that you can just add type="hidden" to the field in your HTML code. With some additional development work, you could then ensure that query parameters in the link to the subscribe form are added to that hidden field. They wouldn't need to be UTM parameters specifically, any parameters you create would work okay.
There is a bit of extra development work, so it may be worth looking at a tool like Unbounce. This takes some of the work out of processes like this and integrates with Campaign Monitor :)
Hope that could work for you?