Try these steps:-

1. Create an org-wide email address. Go to **Setup** --> **Organization-Wide Addresses** and add the email address and verify it.

2. In the apex code, add the following:- 
   

        List<OrgWideEmailAddress> lstEmailAddress=[select Id from OrgWideEmailAddress WHERE Address=['your org wide email address']];
        
        Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
        mail.setTemplateId(et.Id);
        mail.setOrgWideEmailAddressId(lstEmailAddress[0].Id);
        mail.setToAddresses(new List<String>{'receiever email address'});
        mail.setTargetObjectId(UserInfo.getUserId());
        mail.setWhatId(objRegistration.Id);
        mail.setSaveAsActivity(false);
        mail.setUseSignature(false);
        List<Messaging.SingleEmailMessage> allmsg = new List<Messaging.SingleEmailMessage>();
        allmsg.add(mail); 
        
        Messaging.sendEmail(allmsg,false); 


Hope this helps.
Thanks !!