SMTP/MX Verification

Websitehttps://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol
CategoryLive Runtime Lookups

SMTP/MX Verification performs live checks against mail servers by resolving MX records and establishing SMTP connections. This verifies whether a domain's email infrastructure is reachable, properly configured, and responsive. We use these live verification checks on robtex.com and dns.ninja to provide real-time email deliverability assessment.

Source:SMTP/MX Verification

What is SMTP/MX Verification?

Email delivery on the internet relies on a chain of DNS records and server connections. When someone sends an email to user@example.com, the sending mail server must:

  1. Query MX records - Look up example.com's MX (Mail Exchanger) DNS records to find the designated mail servers, each with a priority value
  2. Resolve mail server addresses - Look up the A/AAAA records for the MX hostname to get the server's IP address
  3. Establish SMTP connection - Connect to the mail server on port 25 (or 587/465 for submission) and perform the SMTP handshake
  4. Exchange greeting - The server responds with a banner identifying itself and indicating readiness to receive mail

Our verification process follows this same chain, checking each step for proper configuration:

  • MX record existence - Does the domain have MX records at all? Without them, mail delivery relies on fallback A record delivery, which is unreliable
  • MX resolution - Do the MX hostnames resolve to valid IP addresses? Dangling MX records that point to non-existent servers are a common misconfiguration
  • SMTP connectivity - Can we establish a TCP connection to the mail server on port 25? Firewall rules, server downtime, or network issues may prevent connections
  • SMTP banner - Does the server respond with a valid SMTP greeting (220 status code)? Some servers are reachable but misconfigured, returning error codes or invalid responses
  • TLS support - Does the server advertise STARTTLS for encrypted mail transmission? Modern mail servers should support encryption

This verification does not send actual email. It only performs the initial connection and handshake phases that any sending mail server would perform when attempting delivery.

How We Use This Data

On robtex.com and dns.ninja, domain lookup pages display email infrastructure status based on live MX verification. When you look up a domain, we check whether its MX records are valid, whether the designated mail servers are reachable, and whether they respond correctly to SMTP connections.

This helps users quickly assess a domain's email posture:

  • Domain administrators can verify their mail servers are reachable from external networks and responding correctly
  • Security analysts can check whether a suspicious domain has functional email infrastructure, which may indicate its level of operational sophistication
  • Email deliverability troubleshooting - When email to a domain bounces or is delayed, checking MX records and server connectivity is the first diagnostic step

We display the MX records with their priorities, the resolved IP addresses, and the SMTP connection status for each mail server. TLS support is noted where detected, as encrypted mail transport is increasingly expected.

FAQ

Does this verification mean email to the domain will actually be delivered?
Not necessarily. Our verification confirms that the mail infrastructure is reachable and responds to initial SMTP connections, which is a necessary but not sufficient condition for delivery. The receiving server may still reject specific messages based on sender reputation, SPF/DKIM/DMARC policies, content filtering, or recipient validation. A successful verification means the plumbing works; actual delivery depends on many additional factors.
Why does a domain have no MX records but still receives email?
RFC 5321 specifies that if no MX records exist, the sending server should fall back to the domain's A record and attempt SMTP delivery there. This works in practice but is not recommended because it bypasses the explicit designation of mail servers that MX records provide. Some small domains rely on this fallback, but it can cause reliability issues because there is no way to specify backup servers or priorities without MX records.
Can SMTP verification detect whether a domain is a spam source?
SMTP verification alone cannot determine if a domain sends spam. It only checks whether the domain's inbound mail infrastructure is functional. However, the results can provide contextual clues: a domain with no MX records or unreachable mail servers that is actively sending email is suspicious, as legitimate domains typically maintain working inbound email. For spam assessment, other signals like DNSBL listings, SPF/DKIM configuration, and domain age are more relevant.