This error code, and the URL referencing login.xfinity.com, indicates that the issue is with your SMTP server settings and how it's attempting to authenticate your account.

Open Thunderbird's Account Settings: Go to Tools > Account Settings.

Select Your Email Account Xfinity, Gmail, or AT&T that is having the issue.

Find the SMTP Server Settings.

Edit the Server for the account you're working on (e.g., smtp.comcast.net for Xfinity) and click the Edit button.

Verify Authentication and Port Settings Check the correct port number for your SMTP server.

       Xfinity: 587, Gmail: 465 or 587, AT&T: 465

Connection security should be set to SSL/TLS or STARTTLS

Authentication method should be set to Normal password or OAuth2

The user name for the SMTP server is usually your full email address

After making these changes, click OK to save. try sending a test email from the account again.

For Xfinity, assuming you have already enabled access by 3rd-party apps like TB, check your outgoing settings:

smtp.comcast.net, 465, SSL/TLS, normal password, email address

https://www.xfinity.com/support/articles/update-your-xfinity-email-settings

For AT&T, you must use a secure mail key in place of the account password.

https://www.att.com/support/article/dsl-high-speed/KM1010523/

Gmail should be straightforward, with the automatic settings.

If you've done all this, run Windows in safe mode to bypass startup apps like AV, VPN etc. that might be blocking sending.

Xfinity setting you showed same as I use on all machines. Not working in TB,

GMail setting same as Xfinity except smtp.gmail.com. Same as I use on all machines. Not working TB.

ATT uses smtp.mail.yahoo.com with other settings the same and unique secure mail key for every machine. Not working in TB.

I run Norton 360 and the VPN is turned off.

Run Windows in safe mode or uninstall Norton (disabling it is not sufficient).

George — When I go to Server Settings, I don't see anything for the outgoing SMTP server. Only thing showing in the incoming POP server. How do I get to the SMTP settings? They only showed up when I was setting up the account the first time.

Sfhowes — I could not get to Safe Mode. After looking up how to get to safe mode in Win 11, I ran into BitBlocker. After more research to find the recovery key and got past there, it said my Pin was missing and I needed to set it u[p again, but it would never go past that screen to let me set it up. A whole lot of time and frustration nothing gained.

In Account Settings, bottom of left pane, Outgoing Server (SMTP).

If you can't run Windows in safe mode you could temporarily uninstall Norton. Or, disable all Norton settings that affect TB and the TB profile folder. Search this forum for several topics related to Norton 360.

IFNorton 360 and TB are conflicting, are there solutions for getting TB to work with Norton 360? Do I need to look for a different solution?

I was switching from Outlook to TB because Microsoft 365 only downloads the "New" Outlook that does not support POP3 accounts. "Classic" Outlook can still be downloaded, but long-term support is unknown.

Here's how you can find and edit the SMTP (outgoing) server settings in Thunderbird:

Step-by-Step Instructions

Open "Account Settings": Go to the Thunderbird menu bar at the top of the window. Click on Tools and then Account Settings.

Locate "Outgoing Server (SMTP)": In the left-hand pane of the "Account Settings" window, scroll down the list of accounts. At the very bottom, below all of your email accounts, you will see an entry called Outgoing Server (SMTP). Click on this.

Find and Edit Your Server: In the right-hand pane, you will now see a list of all the outgoing (SMTP) servers that have been set up in Thunderbird. Each of your accounts should have its own entry here.

Select the server you want to edit (e.g., the one for your Xfinity, Gmail, or AT&T account).

Click the Edit button.

Verify the Settings: A new "SMTP Server" window will pop up. This is where you can confirm or change the settings for your outgoing mail. Double-check the following for each account:

The correct address for the outgoing server.

Xfinity: smtp.comcast.net Gmail: smtp.gmail.com AT&T: smtp.mail.yahoo.com (as you correctly noted, AT&T email often uses Yahoo's servers)

The correct port number Xfinity: 587, Gmail: 465 or 587 (465 is for SSL/TLS, 587 is for STARTTLS), AT&T: 465 or 587

Connection Security: This should be set to SSL/TLS or STARTTLS depending on the port.

Authentication Method: This should be OAuth2 for modern accounts like Gmail or Normal password for most others.

User Name: Your full email address.

Save Changes: After making any necessary changes, click OK to save the new settings. Then, click OK again in the "Account Settings" window to close it.

Since you are receiving messages but not sending them, the problem is almost certainly with the outgoing SMTP server settings, but other factors could be at play.

You mentioned you use Norton 360. While you have the VPN turned off, your antivirus or firewall software might still be blocking Thunderbird's connection to the outgoing mail servers. Try temporarily disabling Norton 360's firewall or email scanning feature and see if you can send a message. If that works, you will need to add Thunderbird as a trusted application in Norton 360's settings.

For services like Gmail and AT&T (which uses Yahoo), if you have Two-Factor Authentication enabled on your account, you cannot use your regular account password in Thunderbird. You need to generate a specific "app password" from your email provider's website. This unique, one-time password is what you will use for the password in Thunderbird's settings. You would need to do this for each account.

The error message "Access Denied... login.xfinity.com" strongly suggests an authentication problem. This means that while Thunderbird is connecting to the correct server, the server is denying your login attempt. This could be because of:

An incorrect password.

The need for an app password (if 2FA is on).

A server setting that is not correctly configured to accept your login credentials (e.g., incorrect authentication method selected in Thunderbird).

I hope this helps you get your emails sending again!

IFNorton 360 and TB are conflicting, are there solutions for getting TB to work with Norton 360? Do I need to look for a different solution? I was switching from Outlook to TB because Microsoft 365 only downloads the "New" Outlook that does not support POP3 accounts. "Classic" Outlook can still be downloaded, but long-term support is unknown.

Search this forum for Norton setup options. Or, save yourself the effort and simply remove it and use the built-in Windows Security in Win 10/11.

The 'new' Outlook app on my W11 PC supports POP.

Thanks guys. Finally got all 3 accounts working. It was all two-factor authentication and security key issues. All the account setting I had entered were correct.