I have my email setup on my iphone and can only send messages from that email account if I turn off the Wi-Fi on my phone so it's not using the Spectrum router at home. Sending from my computer I can't turn off the connection to send an email.

My guess is some loon as spectrum is refusing mail from home internet IP addresses to fight spam. It is the sort of thing the technically challenges folk in some ISP's do, they forget they have customers who send mail using something other than web mail, so the mail originated on a home IP address on their own network is denied.

Go to this site https://check.spamhaus.org/ enter the IP address of oyur home router on the sectrum network and check it. I will guess it is blocked. But it will give you something to talk to the script readers about while they connect you to a real technician.

https://check.spamhaus.org/ Reports no issues.

What OS are you using? Have you just upgraded to Windows 11 ? What Anti-virus are you using ? Is 'Thunderbird' program set up as an allowed program in the Firewall?

The spectrum router clearly has a problem. Try this info: https://mobile.spectrum.com/support/article/360001978708/wifi-calling-troubleshooting

I'm on Windows 10 on a Desktop computer. I use Norton 360 Anti-virus. Thunderbird is setup as an allowed program and I've further tested it by turning off the anti-virus and firewall which made no difference. I have no idea why you're posting a link about wi-fi calling. I can't send email from my desktop computer which has nothing to do with wi-fi, I'm hard wired to the internet on my desktop computer.

This is the error message I get when I try too send email from Thunderbird

Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) mail.twc.com was lost in the middle of the transaction. Try again.

I can type the name of the email server listed above and it takes me directly to my webmail login, so I know the email server is correct, but I can't figure out how to fix the message.

I'm on Windows 10 on a Desktop computer. I use Norton 360 Anti-virus.

Norton has a very very long and illustrious career messing up email. Have you tried windows safe mode with networking to eliminate the Norton factor. I suggest you do.

Check the Norton 360 Anti-Virus Under 'My Norton' If the 'Secure VPN' is switched on then switch it off.

I have Norton 360 and it did an update and auto switched on the VPN. It caused me several issues. Thunderbird did not function and I also discovered known good links to end up being redirected to completely irrelevant webpages.

Secure VPN is Off and has been.