Domain of sender address test@example.sg does not resolve

Recently, we decided to move our servers to a new data-center to meet our expansion needs. With the move, our assigned IP addresses changed.

A few days after the move, one of our customers called to highlight that her emails could not be sent to some banks in Singapore. From the logs generated by Sun Java System Messaging Server, we saw the following error:

smtp;451 4.1.8 Domain of sender address test@example.sg does not resolve

We then realized that the PTR record for this customer's domain has not been updated. The problem is fixed once we updated the PTR record to point to our new IP addresses.


How does PTR (aka Reverse DNS Lookup) works in simple terms?

If you configure your email client to send emails via SingNet's MTA, then the IP address of SingNet's MTA(s) must resolve on the receiving party's MTA.

c:\ nslookup
> set type=mx
> singnet.com

Non-authoritative answer:
singnet.com MX preference = 10, mail exchanger = mx16.singnet.com.sg
singnet.com MX preference = 10, mail exchanger = mx17.singnet.com.sg
singnet.com MX preference = 10, mail exchanger = mx18.singnet.com.sg
singnet.com MX preference = 10, mail exchanger = mx11.singnet.com.sg
singnet.com MX preference = 10, mail exchanger = mx12.singnet.com.sg
singnet.com MX preference = 10, mail exchanger = mx13.singnet.com.sg
singnet.com MX preference = 10, mail exchanger = mx14.singnet.com.sg
singnet.com MX preference = 10, mail exchanger = mx15.singnet.com.sg

mx17.singnet.com.sg internet address = 165.21.74.117
mx18.singnet.com.sg internet address = 165.21.74.118
mx11.singnet.com.sg internet address = 165.21.74.121
mx12.singnet.com.sg internet address = 165.21.74.122
mx13.singnet.com.sg internet address = 165.21.74.113
mx14.singnet.com.sg internet address = 165.21.74.114
mx15.singnet.com.sg internet address = 165.21.74.115
mx16.singnet.com.sg internet address = 165.21.74.116

Let's do a reverse DNS lookup on mx17.singnet.com.sg's IP address (165.21.74.117):

c:\ nslookup
> set type=ptr
> 165.21.74.117

Non-authoritative answer:
117.74.21.165.in-addr.arpa name = mx17.singnet.com.sg

It should resolve, otherwise the above error will be encountered.

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