Why does sendmail say "DSN: Can't create output" when receiving an email?

Question

send mail

Hello.
Set up sendmail mail server on Centos.
To collect emails in etc/aliases I wrote:
muser: "/web/sites/.../www/mail".
muser - not a virtual user (tried with a virtual one as well), it is also registered in the sendmail settings. On the specified path configured access to the user and group mail (checked).
But when the server receives letters, it does not save them. Tried sending emails from different mail servers. Log entries var/log/maillog:
Dec 16 06:56:01 mail sendmail[1100]: 1BG3u1bS001100: from=, size=2950, ​​class=0, nrcpts=1, msgid=, proto=ESMTP, daemon=MTA-ext , relay=huan9.mail.rambler.ru [81.19.78.8]
Dec 16 06:56:01 mail sendmail[1101]: 1BG3u1bS001100: to="/web/sites/.../www/mail", ctladdr= (8/0), delay=00:00:00, xdelay= 00:00:00, mailer=*file*, pri=33147, dsn=5.0.0, stat=Can't create output
Dec 16 06:56:01 mail sendmail[1101]: 1BG3u1bS001100: 1BG3u1bS001101: DSN: Can' t create output
Dec 16 06:56:01 mail sendmail[1101]: STARTTLS=client, relay=inmx.rambler.ru., version=TLSv1/SSLv3, verify=FAIL, cipher=ECDHE-RSA-AES256-GCM-SHA384 , bits=256/256
Dec 16 06:56:01 mail sendmail[1101]: 1BG3u1bS001101: to=, delay=00:00:00, xdelay=00:00:00, mailer=esmtp, pri=34171, relay= inmx.rambler.ru [81.19.78.69], dsn=2.0.0, stat=Sent (Ok: queued on 1vm0065.prod.mail.rambler.tech as u11CXQuM-u11a4omx)
But, if you write "muser: root" in etc/aliases, then the letters successfully go to /var/spool/mail.
Other settings:
etc/mail/virtusertable:
@domain.ru muser

If necessary, I can attach all the settings