The upcoming GDPR requirements dictate that from 25th May 2018, marketing must only be sent to customers who have actively opted-in to receive marketing.
So from the 24th, customers who haven't actively opted-in via Appointedd will not be sent your marketing sent via Appointedd.
To give your customers the opportunity to opt-in before they're automatically opted-out on 24th May, all email marketing sent from your account until the 24th will include a link through which the customer can actively opt-in. The link will be worded as follows:
In line with upcoming GDPR requirements, you will be automatically opted-out of marketing from us from 25th May 2018. If you wish to continue receiving marketing from us, click here to opt-in.
We suggest sending at least one email campaign to your customers to make them aware of this change and to give them the opportunity to opt-in to continue to receive your marketing.
On an ongoing basis, customers can opt-in when making a booking or by contacting you (see this guide for full details on how customers can opt-in).