Change the authentication to 'normal password' and the connection security to STARTTLS. If that doesn't work, change the security to SSL/TLS and then manually type in 587 for the port.

https://support.mozilla.org/en-US/questions/1234783#answer-1157050

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

Didn't work. When I changed the ports and security, it wouldn't even log to the server.

The IP reported with the error (small image) is "listed in the CSS" & "listed in the PBL" & "listed in the XBL" - which kind of mean that the IP from where you try to send those mails is "black-listed". You might get in touch with your email server team to set that IP on a white-list or you might try to use another connection (if your provider can offer you).

Wouldn't the IP address of Thunderbird be the same as the internet IP of Spectrum.net? I believe I share one IP address per ISP with all the router devices.

The IP mentioned on that error should be your public IP. Check this information (Google search: what is my IP). The mail server - spectrum - receive a "request" to get mails (from thunderbird) from that IP and refuse the connection if it's black-listed (no matter what email-client you're using) and if that mail server is set like that...