I suggest that a simple, opt-in button be made available for instances where new batches of customer emails are uploaded to the system. As a system user, I recently obtained some customer contact details via a handwritten entry form which had a tick-box for customers to opt in to emails. I want to ensure that we have a digital record of GDPR compliance, so I would like to send out a confirming, opt-in email that allows the customers to click to confirm that they want to continue to hear from us.