What is the issue?

Some inbound email is being rejected with an SMTP response similar to the following:

Mar 1 13:38:21 spamtitan postfix/smtpd[67378]: NOQUEUE: reject: RCPT from example.com[203.0.113.1]: 450 4.1.8 <info@example.com>: Sender address rejected: Domain not found; from=<info@example.com> to=<xyz@example.net> proto=SMTP helo=<smtp.example.com>

What causes the issue?

This is a safeguard to prevent potential spam from getting through to recipients being filtered by SpamTitan. This restriction is triggered when SpamTitan cannot resolve the sender domain to a DNS A-record or a DNS MX-record.

What is the solution?

There are two possible solutions to this issue:

  • This feature can be enabled and disabled globally on SpamTitan DCI/Gateway. To do this:

    1. Log in to SpamTitan as a global administrator.

    2. Go to System Setup > Mail Relay > SMTP Settings.

    3. Turn Reject Unknown Sender Domain to ON or OFF by selecting Enable or Disable as required.

  • If you are using your own DNS resolvers with SpamTitan, you can create a DNS zone for individual sender domains that you'd like to allow on those DNS resolvers. To do this, create the zone and add an A and/or MX DNS record. This bypasses the domain's authoritative DNS nameservers.

Further Info

We recommend having this feature enabled. On our SaaS platforms, it is not possible to enable or disable this feature, as it is a global feature that affects all tenants.

For more documentation, see SMTP Settings.