Check > "Autenticated sender must use adress from their postoffice"
Plesk Panel send notifications or alert using default postoffice and all emails not send , because use ip 127.0.0.1 with local postoffice to sen other postoffice with other domain.
Mail Enable said : 551 Sender address is not valid for your login. Check your email program settings.
I suggest add new security:
>> "Autenticated sender must use adress from their postoffice if postoffice not exist"