This update restricts organizations from sending emails from an unverified email address in the guest user record. Orgs with a verified organization-wide email address aren’t affected by this release update, since the “sent from” email address defaults to the org’s verified email address. This update is enforced with the Summer ‘23 release.
Where: This change applies to orgs that have a guest user in Lightning Experience or Salesforce Classic.
When: Salesforce enforces this update in Summer ’23. To get the major release upgrade date for your instance, go to Trust Status, search for your instance, and click the maintenance tab.
How: When this update is enforced, emails sent from the org using a guest user’s unverified email address are blocked.
To review this update, from Setup, in the Quick Find box, enter Release Updates, and then select Release Updates. Follow the steps for Restrict Emails Sent from the Guest User.
Restrict Emails Sent from the Guest User (Release Update) (salesforce.com)