Ok there have been a lot of these sending issues with spectrum lately and a lot have had to do with the fact they are quite insistent about the user name in the selected outgoing server (SMTP) used to send the mail and the email address.

So lets start with the settings you have in account settings > outgoing server (SMTP). Please post a screen show of you settings for this account and the incoming account settings page that shown which outgoing server that identity will use. Lots of older accounts have a single SMTP server with the provider (cox spectrum) and multiple email addresses used it. This no longer works with spectrum. If you send email from BillyTheKid@Charter.net then the username on the SMTP used to send must match up with the account name for that email address.

Instructions for creating a screen shot here https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem#w_create-a-screenshot-on-windows

Here's the screenshots. Thanks for your help!

The server names to ne used according to your provider are

   Incoming Email Server: mobile.charter.net
   Outgoing Email Server: mobile.charter.net

https://www.spectrum.net/support/internet/spectrum-email-server-settings

They also recommend (require) you to use port 587 to send mail and port 993 to receive mail for an IMAP account such as you have.

Additionally you need to use connections encryption SSL/TLS as well as normal password to authenticate.

Note here you may have further connection issues if you have an antivirus program that scans email and encrypted connections as you provider requires. Microsoft Defender does not scan email.

This is for my desktop MAC where I get my email. (No email on my phone.) So just to confirm - use mobile.charter.net?

I've never bothered to use connections encryption - left it blank (as is). And, I do not know what my password is, as haven't used it in years. Hopefully, I can reset it.

Thanks.