Salesforce now requires both domain-level and user-level verification to send email. Enable users whose email domains you can't verify to send email from your sites. For example, enable people who use public email services such as yahoo.com or icloud.com to send messages from a site.
Where: This change applies to Aura, LWR, and Visualforce sites and Salesforce Sites accessed through Lightning Experience and Salesforce Classic in Enterprise, Performance, Unlimited, and Developer editions.