Send email by guest without login

You can enable guest login and create an Add page of a dummy tables with fields like subject and body. Make sure that guest user has access to that Add page and use it as a contact form.

Once user fills the form, you can send an email using either BeforeAdd or AfterAdd event of this table. It probably also makes sense to add CAPTCHA to that page to prevent abuse.

Valuable tip. Thanks Sergey