Sphere IT Blog

When deploying a new Exchange server, moving ISP or simply changing MX records we need to keep many things in mind with regards to DNS.

Firstly we need to create an A record which points to the external IP address of your mail server.

Next we will create an MX record which references the A record we just created as to where the mail needs to be delivered.

This will now route mail in correctly but there is still some things we need to check.

RDNS (or PTR) is a reverse lookup and should resolve the IP address back to the A record which points to the mail server. If you don’t have an RDNS record you will find that many mail servers which require RDNS as a mandatory requirement to accept mail will block.

The next thing you need to check is that the external IP address that is port forwarded or NAT’ed to the mail server has a mirrored NAT rule. If for example you have a block of IP addresses and your LANs global NAT is set to a different IP address than the mail server, this mail server will use the incorrect IP address to send the mail. For troubleshooting you can find this in the header of the E-Mail on the recipients end.

Keeping these points in mind you should not have a problem with mail flow within your infrastructure.