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.
How is sender verification done
Lookup MX records of ws1.snel.com, if found, use that server to verify, else use the IP address of the server sending the e-mail to verify
Verify with the following sequence:
$ telnet ws1.snel.com 25 Trying 2a00:7b80:3043:1000::34... Connected to ws1.snel.com. Escape character is '^]'. 220 ws1.snel.com ESMTP Exim 4.84_2 Fri, 12 Jan 2018 12:54:33 +0100 HELO ns3.snel.com 250 ws1.snel.com Hello aaa [bbb] MAIL From: [email protected] 250 OK RCPT To: [email protected] 250 Accepted
Replace ws1.snel.com with the hostname of the MX record from the sender domain and replace [email protected] with the sender address you want to verify. Don’t replace ns3.snel.com or [email protected]
If mail to the sender address is accepted we know that the e-mail address exist.
- 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.