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
<firstname.lastname@example.org>: host vk.com[/var/run/cyrus/socket/lmtp] said: 550-Mailbox
unknown. Either there is no mailbox associated with this 550-name or you
do not have authorization to see it. 550 5.1.1 User unknown (in reply to
RCPT TO command)