To expand the awareness of your brand, Magento 2 allows inviting products to friends via email by setting up the Refer Email to a Friend in the configuration. For the easier way, Email a Friend link appears along with an envelope icon instantly on the product page. Just clicking on the link, customers are navigated to the Email to a Friend page, then complete all information of the email. It is possible to send to multiple recipients, however, to avoid the spam, you should set the limit of the sharing product in one hour and the number of recipients per email.
The Refer Email is the great way to help you drive the traffic to your store while save much more money than other promotions. In addition, sharing by customers will make your products trustworthy in the marketplace.
Stores > Settings > Configuration.
Catalog > Email to a Friend.
Email Templates section,
Enablefield, choose Yes to apply email template.
Max Recipientto the maximum number of friends who are on the delivery list of an email.
Max Products Sent in 1 Hourto the maximum number of products which is allowed to email to friends by single sender in 1 hour.
Limit Sending Byfield with 2 ways:
Save Configto finish.
Email a Friendlink is available to tap.
Send Emailif it is ready.
Ref: Magento 2 User Guide
It comes to the end of tutorial: How to setup Refer Email to a Friend in Magento 2.