Email Issue - “On Behalf Of Blackhole”

Question

I am experiencing an odd email issue that I haven't been able to find any answers to. I have a Ubuntu server setup with postfix that uses mailgun to send emails. I'm not using the mailgun API, just setup as the relayhost in the postfix config.

However a client received an email with the following from address (real domains have been taken out):

From: noreply=[e-mail]
[mailto:noreply=[e-mail]] On Behalf Of Blackhole
Sent: Friday, July 11, 2014 2:01 PM

Let me explain those two domains. They originally wanted the from address to be 'site.org', although the server was hosting only 'serverwebsite.org'. My problem is the client didn't want the "On Behalf Of Blackhole" to appear as the from name, understandably and have no clue why it did.

Any ideas on what would cause it to show that? Since this email was sent out, I have been unable to reproduce the issue and I changed the from address to be a @serverwebsite address - I don't know enough about email to know if that could of fixed the name.

If this helps, here is my postfix main.cf config. I have the username, password, dns setup and all that.

    biff = no
    append_dot_mydomain = no
    smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
    smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
    smtpd_tls_CApath= /etc/ssl/certs
    smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
    smtpd_use_tls=yes
    smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
    smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
    smtp_sasl_auth_enable = yes
    smtp_sasl_password_maps = static:email:password
    smtp_sasl_security_options = noanonymous
    smtp_use_tls = yes
    myorigin = /etc/mailname
    smtpd_banner = $myhostname ESMTP $mail_name
    alias_maps = hash:/etc/aliases
    alias_database = hash:/etc/aliases                                                                                                                         
    mydestination = localhost
    relayhost = [smtp.mailgun.org]:587
    mynetworks = 127.0.0.0/8
    inet_interfaces = all
    mailbox_size_limit = 0
    recipient_delimiter = +
    inet_protocols = all
    home_mailbox = Maildir/
    mailbox_command =
    smtp_tls_security_level = may
    smtpd_tls_security_level = may
    smtp_tls_note_starttls_offer = yes
    smtp_tls_loglevel = 1

Thanks!

EDIT: Output of postconf -n:

alias_database = hash:/etc/aliases
alias_maps = hash:/etc/aliases
append_dot_mydomain = no
biff = no
config_directory = /etc/postfix
home_mailbox = Maildir/
inet_interfaces = all
inet_protocols = all
mailbox_command = 
mailbox_size_limit = 0
mydestination = localhost
mynetworks = 127.0.0.0/8
myorigin = /etc/mailname
recipient_delimiter = +
relayhost = [smtp.mailgun.org]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = static:[e-mail]:password
smtp_sasl_security_options = noanonymous
smtp_tls_CAfile = /etc/ssl/certs/ca-certificates.crt
smtp_tls_loglevel = 1
smtp_tls_note_starttls_offer = yes
smtp_tls_security_level = may
smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtp_use_tls = yes
smtpd_banner = $myhostname ESMTP $mail_name
smtpd_tls_CApath = /etc/ssl/certs
smtpd_tls_cert_file = /etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file = /etc/ssl/private/ssl-cert-snakeoil.key
smtpd_tls_security_level = may
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache
smtpd_use_tls = yes