I think this is a simple question with a simple answer.

Our domain, cory.co.uk, reports a DNS mismatch when you do and SMTP test on mxtoolbox.com. I think this is causing issues sending to some addresses/companies.

We have a PTR: record set for that points to mail2.cory.co.uk, and our MX record is set to mail2.cory.co.uk which resolves to, so that all seems correct.

I was told to set the FQDN on our send connectors (Exchange 2010) to match the MX record which I have done, but it has not resolved the issue. I am wondering, as we have a Sophos mail appliance in between the firewall and mail server, if the mismatch is because it is using the name of the Sophos appliance?

In which case, would the solution simply be to set the name of the appliance cluster to mail2.cory.co.uk?

  • what exactly does mxtoolbox warn you about? they do some tests which assume your outgoing HELO/IP/PTR matches your incoming BANNER/MX IP/MX PTR which isn't necessarily true. – Gryphius Dec 13 '13 at 10:57
  • If you were to do an SMTP: on MXtoolbox, it gives two warnings, one SMTP TLS, which isnt an issue, and one SMTP Reverse DNS Mismatch. MXtoolbox say that this means "the PTR for your server IP address (reverse lookup) does not match the DNS name (forward lookup)". I just tried changing the hostname of our sophos cluster to mail2.cory.co.uk, but that caused some serious issues with loopbacks. – James Edmonds Dec 13 '13 at 11:06
  • Should say this is the only server/IP which we send/receive email from/to. – James Edmonds Dec 13 '13 at 11:08
  • "Warning - Reverse DNS does not match SMTP Banner", so mx toolbox thinks your incoming banner should cointain your incoming PTR.. omg. you can safely ignore that. just make sure your sending appliance uses a resolvable,non-generic, fully qualified name in the (outgoing) SMTP HELO. – Gryphius Dec 13 '13 at 11:10
  • I thought that some receiving servers can use this reverse DNS failure, as a scoring method for spam? Since we have a Sophos cluster, each box has a different FQDN, mail.cory.co.uk, mail2.cory.co.uk, sophos-v-appl1.corybros.co.uk, sophos-v-appl2.corybros.co.uk. Since mail is sent via these boxes, is it looking at the hostname of these for the reverse check? – James Edmonds Dec 13 '13 at 11:19

"Warning - Reverse DNS does not match SMTP Banner"

mxtoolbox issues a warning because your incoming SMTP Banner does not contain the reverse DNS (PTR) of your IP. This test does not make sense since receiving mailservers/spamfilters don't check your incoming banner at all.

However, they care about your outgoing FcRDNS ( which seems to be correct) and they may check if your HELO hostname is fully qualified and resolvable. I'd therefore recommend to add A records for all your cluster FQDNs, all pointing to the sending IP.

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