If you have the correct settings (smtp.cox.net on port 465, SSL/TLS, or port 587, STARTTLS, authentication = normal password, User Name = email address), the error is probably due to an incompatible security/AV app that is scanning outgoing mail. What is your app and its mail scan settings?

How do I find out? What apps am I looking for? I have never had to deal with this before.

You might have a program like Norton, Avast, Kaspersky, or just the built-in Windows Defender. In Windows 10, right-click the Start button, Task Manager, More details, and look on the Startup tab for the apps that run at startup.

Thank you, but so far, can't find anything. I removed Windows defender from having access, but didn't solve the problem. I do have McAfee. Could that be the problem? God Bless you for helping!

Yes, McAfee could very well be the problem. I'd recommend removing McAfee and using Windows Defender only, adding the TB profile folder as an Exclusion. The profile folder location is shown in Help/Troubleshooting, about:profiles.

I will certainly give this a try. Thank you so much!

Nope. I uninstalled McAfee and let Windows Defender take over. I still can't send email.

In Tools/Account Settings, Outgoing Server (SMTP) at the bottom of the left pane, what are the settings for smtp.cox.net? No need to expose your email address.

Also, delete the passwords from Tools/Options/Security/Passwords/Saved Passwords, restart TB, and enter the password when prompted.

smtp.cox.net Port 587 SSL/TLS

There isn't a "security" option under Tools in the upper right corner. There isn't "security" under options (which isn't under tools).

The security on port 587 should be STARTTLS.

Click the icon in the upper right corner, Options, Options, Security, Passwords, Saved Passwords.

Cox specifically sent me SSL/TLS; however, just in case they made a mistake, I changed it to STARTTLS. Attached is the error message I got. It seems much worse!  :-)

I found the saved password and I deleted it.

BTW - I have never heard of Spamhaus.org and have never used it. I don't show it installed, either.

Incorrect greeting means that something is wrong with your settings. If you want help, you have to provide all the information, such as authentication method, User Name format. If Cox says the security is SSL/TLS, then try port 465.

I have tried Port 465, but it didn't work, so I put it back to what they told me to do.

What now? I have done everything you have asked.

I don't have any more suggestions, so perhaps someone else will provide some input.

Okay. Thanks, though, for everything. Hopefully, someone else will see this and answer. I am a little desperate.