I have got a response
I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. For further assistance, please send mail to If you do so, please include this problem report. You can delete your own text from the attached returned message.
The mail system Failed to deliver to Recipient XXXX@server
---------- Forwarded message ---------- From: MMMM@server.com To: XXXX@server, YYYY@server Subject: kind message
It says that I cannot deliver to that email. I can also conclude that email can be delivered to the first. But having capacity is not the same as using it. I read response as "such and such problem does not allow us to execute the request". It is reasonable to deliver everything or nothing for transactional systems. In result, I cannot understand if the mail was actually delivered to the second recipient or not?
I see that both mail.yandex.ru and gmail.com are identically uncertain when this happens. Is it a convention?