Yes and no.
Enabling a user to forward an eMail to a colleague, friend or family member using a function that you have included in the eMail is legal.
Then, what isn’t? The controversy comes from the use that is made of the forwarded information.
When you use a provider to send campaigns, you can include the option Send to a Friend. This sends you to a landing page, where you can write the name of the users that you want to forward the eMail to and another field in which you can write comments. Technically you will have access to this data, but not legally, because they have not given you their express consent to send them commercial messages.
To avoid losing this valuable data and to enable you to enrich your database with quality prospects, you should include the possibility of signing up to your newsletter in the design of your campaign by directing the user to the exact section where they can sign up.
Use these tricks to increase your database. Greetings to all!