GDPR requires that you get an affirmative and provable consent to send EU citizens marketing emails. You can do this by obtaining a double opt-in from them.
What is a double opt-in?
A double opt-in is a confirmation of a subscriber’s wish to receive your emails.
To obtain the subscriber's confirmation, you must:
If the subscriber visits the link, then they are 'double opted' in, which confirms that they have agreed to receive marketing emails. This ensures that you can legally send your marketing emails to the subscriber.