If your users are receiving a #550 4.4.7 QUEUE.Expired; message expired ## NDR, it is possible that your mail server’s connection to the recipient is being blocked.

What does “#550 4.4.7 QUEUE.Expired; message expired ##” mean?

You may find that if you try and telnet to the recipient’s mail server on port 25, you get a message along the following lines:

“Your connection has been blocked due to your mail server’s poor reputation or invalid PTR record”

Some mail servers will drop the connection without any message at all.  However if you get the “could not open connection” error, it is more likely that the recipient’s server is currently experiencing issues.

The first thing to do is check that your SMTP server is not in a spam database. To do this first visit http://www.dnsbl.info/ and enter the IP address your email is sent from.

This will give a list showing if you are on any spam databases. If you are, you will need to contact the database owner to get your IP address removed. The other possible issue is your PTR record (reverse DNS) may be missing or it may be generic.

To check your PTR record or find your public IP address visit http://supergeek.co

If your PTR record is missing or looks like an IP address with hyphens then you will need to ask your domain name provider to have the PTR DNS record changed to something more specific.  When changing this, you will want it to become an (A) record that points to your mail server e.g. mail.example.com

Mail acronyms used in this article

What is NDR? — A Non-Delivery Report
What is PTR? — A Pointer Record
What is DNS? — Domain Name System
What is SMTP? — Simple Mail Transfer Protocol