How is verification done?

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We enforce sender verification on our relay server. In short it means the sender should be an existing e-mailaddress. The long version is that under some circumstances even an existing e-mail address could fail the verification.

Let's say in our example mail is sent from and we want to check the existence of the address

How is sender verification done

Step 1)

Lookup MX records of, if found, use that server to verify, else use the IP address of the server sending the e-mail to verify

Step 2)

Verify with the following sequence:

$ telnet 25
Trying 2a00:7b80:3043:1000::34...
Connected to
Escape character is '^]'.
220 ESMTP Exim 4.84_2 Fri, 12 Jan 2018 12:54:33 +0100
250 Hello aaa [bbb]
MAIL From:
250 OK
250 Accepted

Replace with the hostname of the MX record from the sender domain and replace with the sender address you want to verify. Don't replace or

Step 3)

If mail to the sender address is accepted we know that the e-mail address exist.

Known issues

  • Sender verification on Office 365 platform is not allowed. Please use O365 SMTP server when sending mail with O365 addresses.
  • When an external forward is setup on some control-panels sender verification could fail. Check with above steps. Use another e-mailaddress as sender.
  • If the IP address you are connecting from is listed on a blacklist the mail could be rejected and the verification could fail. Make sure that you test from a location which is not listed on a blacklist. Use an online tool to verify that your IP address is not listed on a blacklist.
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