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[ietf-smtp] MX Priority and Microsoft/GMail adherence

2016-02-20 18:34:44
Hi there, just joined to try and get some knowledge/help on an issue when 
getting emails delivered from (particularly) Hotmail/ but 
occasionally Gmail addresses.  I consulted RFC5321 and it does say the mail 
delivery will be tried in order of MX preference.


We have set up a Sophos UTM which is a pretty sophisticated device.  This is in 
front of an Exchange server and filters all incoming/outgoing emails.

The domain being used is, say,

Our MX records for this domain are:




The 3rd priority MX record is in the event of an outage, the emails queue and 
return to the top MX entry once the mail server returns from e.g. a reboot or 
Internet downtime.  It is hosted in a different country to the top priority MX 

What we are finding is that 90% of Hotmail/ emails sent to the 
domain are coming from  All other domains behave 
as expected and come in via 0  Some Gmails follow this 
behaviour too.

This is where the IETF comes in as it would appear to go against the RFC for MX 

What brought this to our attention was that our Sophos UTM instantly started 
rejecting emails from our 3rd party MX provider, all of them from 

It is happening with all 3 of the UTM devices that we have fitted so far, all 
on different domains, different public IP addresses, all in Gibraltar.  Our 3rd 
Party MX is located in Manchester, United Kingdom.

We have obviously now added our 3rd Party as an upstream relay but this is not 
ideal – why are these emails going to the 3rd priority MX record in the first 

The primary MX public IP addresses on all 3 UTM's are not blacklisted and 
pretty much have 99% or higher availability.

I am going to be asking Sophos if the UTM is in any way delaying it’s response 
to the Microsoft mail servers to eliminate it being the UTM taking too long to 
reply for Hotmail’s/Gmail’s liking.

I do not think that is the case as it is only with these two email providers 
that we have the issue.

Thanks in advance for any possible help or advice.



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