Sender Verify for Receiving Email

In order to restrict the quantity of bogus and spam messages our server requires that every inbound mail message be sent from an email address which can be verified as genuine. If the server is unable to send an email (or a bounce message) back to the address that the message appears to come from, the message will not be accepted and therefore not delivered.

If you receive complaints that email messages are being rejected by the server, and the error message mentions sender verification, please ask them to contact their mail administrator (e.g. the postmaster) to ask them to follow this up and suggest they read the text below. Also if the sender is unable to receive mail, for example if their account isn't working or is temporarily down or over quota, they won't be able to send you mail if sender verify is enabled.

To check rejected messages for a particular user run:

root@server30013 [~]# grep -inw 'sender verify'
/var/log/exim_mainlog | grep 'chris@pim-uk.com'

Technical notes for mailserver adminstrators

To verify a message's sender address, our mailhub will attempt the following...

• check that the header is syntactically correct
• attempt to find a mailserver for the domain part of the sender address (based on either MX or A records)
• open an SMTP connection to that server
• send the following:

  HELO our-server-name
  wait for success response
  MAIL FROM: <>
  wait for success response
  RCPT TO: sender-address-from-your-message
  wait for success response
  QUIT

If any step fails, the message will be rejected.

Note that the dummy mail is sent as if it were a bounce receipt, thereby testing whether a bounce message is deliverable to that address.

If your server does not allow bounce receipts, it is not functioning correctly.
Please read RFC2821 (in particular, sections 4.1.1.2 and 3.7) and ensure that your mailserver complies with the RFCs before complaining to us.

If you are certain that your mailserver is behaving correctly, please email me with a detailed fault report, including the full text of the bounce message and its full SMTP headers.

In the future I will setup a postmaster address which will not be sender-verified so you may be certain that your report will arrive unchecked.