Usually the word 'relay' is used when an email client is sending from one account, yet using the SMTP server of another. Possibly if you post screenshots of incoming server settings pane, and one of the SMTP server settings pane, someone can advise. Also, do you have more than one account within that domain name?

It doesn't matter which smtp server I use

That might be part of the problem if you're referring to accounts that use the same server. Until we see the server information and know how many accounts use the same server settings, it will be difficult to advise.

/Users/davidharris/Desktop/Screenshot 2025-04-09 at 1.45.41 PM.png

Description: Charter Server Name: mobile.charter.net Port: 465 User Name: davidfharris@charter.net Authentication method: Normal password Connection Security: SSL/TLS

Try port 587

Send Message Error Sending of the message failed. An error occurred while sending mail. The mail server esponded: Non-Compliant sender address. Code: -1. Please check the message and try again.

please post a screenshot of the account pane pane, as that may give more info on the email address.