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Misconfigured SMTP servers (general)

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Misconfigured SMTP servers (general)

To ensure uninterrupted email delivery, it is critical that all technical settings be correct. In case of difficulty, they should be double checked.

  • Misconfigured SMTP servers may have problems delivering email to many networks, including ones that do not use Spamhaus data.

SMTP servers should be configured with correct forward and reverse DNS and HELO/EHLO values.

  • DNS is used to resolve a domain name to an IP address. This act is known as a forward DNS and is performed every time you visit a site on the internet. Reverse DNS (rDNS) is a method of resolving an IP address back to a hostname. Your hosting company or email service provider can help set this up or correct it.
    • If you need to correct your rDNS and are using OVH cloud or Amazon cloud (Lightsail) they have information on their websites that can help.
    • The forward DNS lookup (domain name to IP address) of your IP should match the HELO value set in your server.
  • HELO is an SMTP command sent by an email server to identify itself when connecting to another email server, to start the process of sending an email. It is followed with the sending email server’s hostname.
    • One way of testing whether an SMTP server is misconfigured is to send an email from it to [email protected]. A bounce will immediately be returned that contains the required information.
    • Common misconfigurations of HELO are: “localhost.localdomain”, “WIN-XYZPDQ01” or “mail”.
    • A HELO/EHLO value should be a fully qualified domain name (FQDN)
  • About loadbalancers:
    • Some sites place their mail servers behind load balancers. While this may have some advantages, this practice has the side-effect of giving the public-facing IP the appearance of being infected by malware. If this is the issue, then the solution may require an engineering design change.

NOTE: If the IP is not a mail server, or the HELO settings are in fact correct, check for malware or a misconfigured process. You may need to contact your IT department, network engineer, MTA vendor or hosting company for help.

 

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