Monday, September 10, 2018

It's somewhat common check if a sender domain exists before accepting mail. The email RFC has a section that specifically addresses sanity checks for validating the From: addresses in the SMTP transaction. 

https://tools.ietf.org/html/rfc2505#section-2.9

We are changing the way that our system functions as of today andwe will be deferring emails from unknown Domains or domains that return NXDOMAIN to a DNS queries. Historically we accepted these messages.

Common Questions

What if there is a DNS problem resolving the sending domain?

• If this DNS failure is transient the sending SMTP server should retry sending the message to our gateways and we will attempt to resolve the DNS record at that time. 

What common problems have you seen?

• During our testing we have noticed issues with customers who have systems with undefined host names sending alerts to themselves. They often use domains like mycompany.local or www@data which are not valid DNS names. These messages will now be deferred unless they are whitelisted.