My clients have me use a variety of tools for sending out e-newsletters. Some have enormous lists of their clients, some have thousands of people who have subscribed through their site. Since we don't spam, EVER and don't buy lists, I was curious why one particular list had so many returns.
Our list dates back seven years, so many things change. To better analyze this anomaly, I actually manually glanced through each and every of the 2500 that came back. Usually we just delete en-mass, or let the email marketing program handle it all, but I wanted to know because it was such a large number.
Each returned email has an error code, a couple of them may look like this:
Failed Recipient: gardner@yahoo.com
Reason: Remote host said: 421 4.7.0 [TS01] Messages from 209.161.34.182 temporarily deferred due to user complaints - 4.16.55.1; see http://postmaster.yahoo.com/421-ts01.html
This could mean:
| 421 - [ESMTP server temporarily not available] | Our systems are experiencing an issue which is causing a temporary inability to accept new messages | Retry your mailings at a later time |
| 421 - [too many sessions from <source IP>] | The sending IP has exceeded the maximum concurrent connection limit, currently 5 | Configure your sending server to establish no more than 5 concurrent connections |
| 421 - [<source IP> DNS check failure - try again later] | The reverse DNS check of the sending server IP address has failed | Ensure that your DNS configuration is correct and that the PTR record is resolvable |
or
Failed Recipient: gedcool@gamil.com
Reason: Remote host said: 550 5.1.1 <gedcool@gamil.com>: Recipient address rejected: User unknown in virtual alias table
This could mean one of two things:
| 550 - [<mailfrom address> sender rejected] | Cox requires that all sender domains resolve to a valid MX or A DNS record | Ensure that your domain has a valid MX or A record within DNS |
| 550 - [<recipient email> recipient rejected] | The intended recipient is not a valid cox.net account | Ensure that your mailing list is up-to-date with verified recipient addresses |
The first number is key to understanding the reason for the message not being delivered. Then, you can decide whether to change your setup, remove that person from the list or correct other issues.
I found this article from Comcast helpful and wanted to pass it along.
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