This is the mail system at host eforward3e.registrar-servers.com.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients.
<mailadmin@mydomain.com>: unknown user: "mailadmin@mydomain.com"

Delivery status:

Reporting-MTA: dns; eforward3e.registrar-servers.com
X-Postfix-Queue-ID: 94B692E029B
X-Postfix-Sender: rfc822; SRS0+HIah=25=yandex.com=my-email@eforward3e.registrar-servers.com
Arrival-Date: Mon, 20 Mar 2017 11:34:14 -0400 (EDT)

Final-Recipient: rfc822; mailadmin@mydomain.com
Original-Recipient: rfc822;mailadmin@mydomain.com
Action: failed
Status: 5.1.1
Diagnostic-Code: X-Postfix; unknown user: "mailadmin@mydomain.com"

smtpd_banner = $myhostname ESMTP $mail_name (Ubuntu)
biff = no
append_dot_mydomain = no
readme_directory = no
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtpd_relay_restrictions = permit_mynetworks permit_sasl_authenticated defer_unauth_destination
myhostname = mydomain.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = mydomain.com, localhost.localdomain, localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 127.0.0.1/24
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all
inet_protocols = all
home_mailbox = Maildir/
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_local_domain =
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_sasl_auth_enable = yes
smtpd_recipient_restrictions =permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination
smtp_tls_security_level = may
smtpd_tls_security_level = may
smtp_tls_note_starttls_offer = yes
smtpd_tls_loglevel = 1
smtpd_tls_received_header = yes