Hi Steve,

Check if"Allow All Profiles to Use this From Address" in Organization-Wide Email Addresses is selected on profile.

Can you check API version of controller if you have any?    

https://help.salesforce.com/articleView?id=emailadmin_orgwide_addresses_considerations.htm&type=5

Set up a dedicated email address by navigating to Setup -> Administration Setup -> Email Administration -> Organization-Wide Addresses menu. Once you have created an org-wide address (note that Salesforce will require you to confirm the address prior to using it, so if you're going to be sending things from a junk address it would be wise to set up a catch-all mailbox so you receive the confirmation email), grab the Id from the URL and use the setOrgWideEmailAddressId(Id) method on your instance of Messaging.SingleEmailMessage.

If you want to avoid hard-coding an Id, after creating your Org-Wide Address you can query them:

OrgWideEmailAddress[] owea = [select Id from OrgWideEmailAddress where Address = 'doNotReply@blahblah.com'];
Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
if ( owea.size() > 0 ) {
    mail.setOrgWideEmailAddressId(owea.get(0).Id);
    
    }
    
Hope above information was helpful.

Please mark as Best Answer so that it can help others in the future.

Thanks,
Vinay Kumar