I run an app with around 10,000 users. When users register, we show them a checkbox saying "Send me occasional tips and updates", which is unchecked by default. If users don't check it, we don't send them marketing mail.
I would like to email about 20 users personally, a bespoke email from my own account, to ask them a question about something they did on the app.
I would also like to send a survey to a larger number of users, using Mailchimp.
Both of these will be for the purposes of improving the product.
Under GDPR, am I allowed to do either or both of these? I'm not clear either this would fall under the scope of GDPR; and if so, whether any of my users would be viewed as having consented to this kind of research.