Why don't letters reach rouncube?

Question

Mail server

Hello, I installed roundcube, I'm trying to send a letter to it, nothing comes up.
in response I receive the following letter:
Subject: Undelivered Mail Returned to Sender
Message:
This is the mail system at host domen.ru.
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 postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
(expanded from ): unknown user:
"domen.ru/reply/@domen.ru"