With Summer 23 release, Salesforce enforces Guest Site User email to be verified. Without the verification, the Guest Site User will not be able to send any emails anymore. This will impact application confirmation emails, along with other possible configuration.
In order to verify the email address you need to set up this email address as a (or one-off) org-wide default.
Use the following steps:
Setup → Organization-Wide Addresses → Add address of the Guest Site User with profile (any) selected (see screenshot below) → verify via link you get on your email.
More information on site guest user you can read here: https://help.salesforce.com/s/articleView?id=sf.orgwide_email.htm&type=5