Member Avatar for geofoxer

Hello all,

I am wondering how email servers usually treat an email that has no real email domain from which it originated in comparison to an email that has a real email domain such as . How do email servers treat such emails when they are received. For example, some programs, such as nagios, generates emails through which users can be notified of server problems. The email looks like this when it is received: User for Nagios [te]. Although, you can not send an email back to this address becuase the domain does not exist. My question is how most email servers would treat such emails (perhaps as spam) if they were sent on a regular basis.

Thanks for the help ahead of time.

[EDIT: my bad if this post does not belong here]

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Dani AI

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Short summary and practical steps based on the thread: receiving MTAs use several checks beyond the printable From: header. These include DNS lookups (MX/A/PTR), SMTP HELO/EHLO checks, SPF/DKIM/DMARC results, RBLs/blacklists, greylisting and rate limits. As noted, policy varies by server—some reject at SMTP time, others accept then tag or drop messages—so behavior depends on the recipient system.

What to do for alerts and monitoring (actionable checklist):

  • Ensure the envelope sender (SMTP MAIL FROM / Return-Path) is a routable mailbox in a domain you control, not a non-existent host-local address.
  • Publish DNS records: an A (or AAAA) and/or MX for that domain, an SPF record that authorizes your sending IP or relay, and optionally DKIM signing and a DMARC policy.
  • Make the sending host have a PTR (reverse DNS) that resolves to a sensible hostname and matches forward DNS where possible.
  • Use an authenticated smarthost if your IP is in dynamic/residential ranges.
  • Provide a real mailbox for bounces (don’t rely on a non-deliverable reply address).

Postfix example (rewrite local sender to a valid domain):

# /etc/postfix/main.cf
myhostname = monitor.example.com
myorigin = example.com
sender_canonical_maps = hash:/etc/postfix/sender_canonical

# /etc/postfix/sender_canonical
nagios@localhost    nagios@alerts.example.com

Then run postmap /etc/postfix/sender_canonical and reload postfix.

Troubleshooting tips:

  • Reproduce delivery over SMTP (telnet or swaks) to see 4xx/5xx responses at send time.
  • Inspect received headers on delivered messages to check SPF/DKIM/DMARC outcomes and which hop rejected or tagged the mail.
  • If alerts are frequent, watch for greylisting and rate-based throttling.

This approach prevents most rejections and reduces the chance monitoring alerts are treated as spam. ’s case will be solved by giving alerts a proper, routable sender and the relevant DNS/authentication records.

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Member Avatar for DimaYasny

that mainly depends on the email server's setup. a normally setu up MTA will never even process an email that doesn't originate from a valid domain, and is sent from a SMTP server that has no mx record

Member Avatar for geofoxer

Ok cool, thanks a lot for the help!!!

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