Block Emails Sent from Unverified Email Addresses in the Guest User Record

To continue sending emails from the guest user record, use an organization-wide email address (OWA) as the sender. Follow these steps:

1. Create an organization-wide email address if your org does not already have one.
2. Verify the organization-wide email address.
3.  Add the organization-wide email address in the Email field of the guest user record. If you use Apex to send emails from guest users, set the sender to the verified org-wide email address using the setOrgWideEmailAddressId() method as shown in the example below.

Query the OrgWideEmailAddress object to retrieve the Id of the desired do-not-reply address, then call setOrgWideEmailAddressId() on the SingleEmailMessage object before sending

Sample code:
 

OrgWideEmailAddress[] owea = [select Id from OrgWideEmailAddress where Address =
'doNotReply@<somedomain>.com'];
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
if ( owea.size() > 0 ) {
mail.setOrgWideEmailAddressId(owea.get(0).Id);
}