With the rollout of Summer’ 23 release, emails going out from Guest User will stop working if the Guest User Email record does not have a verified organization wide email address added to it.
If your emails are impacted as well after Summer’ 23 release, follow these steps to start them again:
Salesforce sets a dummy email address while a new guest user is created, set a valid email Address on the Guest User record.
Add the same email address in your Organization-Wide Email Addresses and verify that address.
If you are using Apex Code to send the email, query the Org Wide Email address and assign it to the SingleEmailMessage object.
List<OrgWideEmailAddress> oeList = [select Id from OrgWideEmailAddress where Address = 'mysite@gvmantra.com'];
Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
if(oeList.size()>0){
email.setOrgWideEmailAddressId(owea[0].Id);
}