I have taken out a hosting agreement with a well respected hosting provider for a couple of internet facing servers. We have deployed several applications to these servers which send various e-mails back to us for reporting purposes.
Each server runs Windows Server 2003 R2 with the IIS 6.0 SMTP service installed.
Each application is configured to use the local instance of IIS to send e-mails.
The external IP address of each server is mapped to a particular domain eg:
These e-mails are sent from a company domain name and not the domain name of the hosted servers (eg: email@example.com)
A small number (<1%) of e-mails sent from these applications appear to be duplicated. These are exact duplicate in terms of both content and message headers.
I contacted my hosting provider and they told me this was a common problem & instructed me to:
- Change the HELO response of your mail server service to a FQDN (server1.mydomain.com && server2.mydomain.com)
- Create a DNS A record that resolves the FQDN of your mail server to the primary IP address of your sending mail server.
- Create a PTR record that resolves your primary IP address back to your mail server's FQDN
- In the sending domain's (mycompanydomain.com) DNS zone file, add the appropriate SPF record for your hosted servers. eg: v=spf1 a mx include:mydomain -all
The Problem Continues:
I made all of the changes as prescribed above, I was a little hesitant because these steps seemed to suggest they were more for stopping your messages getting blocked than they were for stopping them from being duplicated - but I am certainly no expert in these matters.
It has been 5 days since I applied this fix and the problem still persists.
I am certain that these problems are not a bug in the software because they are 4 different applications installed on 2 different servers, all of whom are exhibiting this strange behaviour. This behaviour has also not been seen in our UAT environment.
Were my hosts correct to suggest this fix? If not, does anyone know what could be the cause of this problem?