Reverse DNS does not match SMTP Banner

www.mxtoolbox.com is a great place to check SMTP settings on your mail server. “Warning – Reverse DNS does not match SMTP Banner” is a common warning message almost every IT person facing. If you are looking to remove this issue you came to the right place. Just follow below steps and you will be able to remove the warning from SMTP checking systems such as Mxtoolbox.

As shown in below image “SMTP Reverse DNS Mismatch” warning is appearing because SMTP connection is looking for “EX01.domain.local” FQDN and which is local FQDN of your messaging system. (In this scenario – Microsoft Exchange Server 2010) Since EX01.domain.local is not a public name it is unable to solve the DNS.

Perform a Telnet to your Exchange server and determine the SMTP banner shown.

telnet mail.domain.com 25

As shown in above session, the banner indicates a local FQDN of your mail host. To change this you have to go to your Internet Receive Connector and change the text of the banner.

To get a list of receive connectors with its name and banner, execute below command in Exchange Management Shell.

Get-ReceiveConnector | fl name,banner

To change the SMTP banner execute below command. <Internet Server Name> should be your internet FQDN of email server.

Set-ReceiveConnector “Default EX01” -Banner “220 <Internet Server Name> <Banner Text>“

Note: The replacement SMTP banner text string must always start with 220. As defined in RFC 2821, the default service ready SMTP response code is 220.

Now perform a MX Lookup and you will no longer getting a “SMTP Reverse DNS Mismatch” warning.