If you use the option on the Interact tab, Itch.io sends the email for you. They limit how many emails you can send out, and they include an unsubscribe link so people can stop getting emails from a certain game if they don't want them. Itch doesn't want you to go outside that system and send your own emails directly because then you might violate CAN-SPAM or GDPR, and people might not want to use the site anymore if developers add them to random mailing lists or sell their info.