Why mail won't send?

That reverse DNS check is usually used with connections involving TCP port 25 to submit mail; it’s a check of a peer mail server. That’s a spam-detection test. Most spam engines do not have valid DNS. Most computers running mail clients also do not have valid DNS, but do have authentication for the mail server; credentials

It’s fairly rare for a mail server to check a peer host when sending, but there are a few that are configured to do that. I’m going to,assume that’s not happening here, and that all addresses for sending mail that have been attempted here have failed.

Which means... More commonly...

If you’re configured and submitting mail to your mail server via TCP port 25, switch to submitting mail on TCP 465 or TCP 587; whichever port Cox supports/requires.

Also check... Mail clients have two sets of credentials and settings. These settings include the mail server host name and the mail server port and the user and password, as well as a security setting (TLS/SSL connection security, and that’s usually enabled in recent years), among other details. One set for sending via SMTP or ESMTP. Another set for receiving via POP or IMAP.

Mail clients will not send to the mail server when the credentials or settings for the mail server are INCORRECT.

Your settings for SMTP or ESMTP are INCORRECT, or Cox is having problems with their mail server. Most likely, the former.

There is no error code like this AUP#CXSNDR listed.

Actually there was. It is the part after the # that you would try to match to the table provided on this site. It appears that your email client is either using an incorrect DNS server or one is not configured on your computer. You can check to see by going to: System Preferences > Network > Ethernet or Wi-Fi (depending on how you connect your iMac for network access) > Advanced... > DNS tab.

The issue is with Cox email server trying to find the public IP address to send the message to. They attempted to perform a Reverse DNS lookup (which looks up what IP address is associated with the recipient ISP's domain name) to locate them ... and that is what failed.

Thanks for the detailed clarification of when a reverse DNS communication is involved with a mis-configured email client. I really appreciate it. I typically do data captures with Wireshark to better understand the processes that go on in the background, but never thought of doing one with a mis-configured email client. Looks like I will be adding that to my "to-do" list.

Tesserax: FWIW, if you’re getting a decent Wireshark dump of the network traffic, you’re either overriding the use of STARTTLS on the SMTP port 25 traffic, or the mail servers you’re working with are (unfortunately) not configured for that.

Most any SMTP and any POP or IMAP contents should be inaccessible, as that should all be SSL’d/TLS’d.

Here’s a decent intro to SMTP AUTH STARTTLS: https://www.saotn.org/test-smtp-authentication-starttls/

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