Ever got an email message saying something like this?
This is an automatically generated Delivery Status Notification.
THIS IS A WARNING MESSAGE ONLY.
YOU DO NOT NEED TO RESEND YOUR MESSAGE.
Delivery to the following recipients has been delayed.
< firstname.lastname@example.org >
The reason for the problem:
4.1.0 - Unknown address error 451-'DNS temporary failure (#4.3.0)'
If you have, then like me, deciphering this is made heaps easier with these two pages: this page at Microsoft, which list the Enhanced Status Codes for Delivery as per RFC 1893 – Enhanced Mail System Status Codes.
Using this, I now know that 4.1.0 & 4.3.0 means:
4.X.X Persistent Transient Failure
A persistent transient failure is one in which the message as
sent is valid, but some temporary event prevents the successful
sending of the message. Sending in the future may be successful.
X.1.0 Other address status
Something about the address specified in the message caused
X.3.X Mail System Status
Mail system status indicates that something having to do
with the destination system has caused this DSN. System
issues are assumed to be under the general control of the
destination system administrator.
X.0.0 Other undefined Status
Other undefined status is the only undefined error code. It
should be used for all errors for which only the class of the
error is known.
All of which let me know where to start looking in order to fix this. Hope you find it useful too.