Please do these checks first:

Firewall Firewall may controlled by the computer default or by an Anti-Virus you installed. Access Firewall and make sure Thunderbird is set up as an allowed program.

VPN Generally servers do not like VPN as they attempt to hide your IP, so server may block if it thinks you are a hacker. Basically switch off VPN or create a split tunneling for Thunderbird to use. Recently many Anti-Virus products are providing VPN - do check they have not auto enabled the VPN

I note that this smtp is using the yahoo server which means you have to use an app generate password. It means your normal password gets you access to your webmail account, but third party apps like Thunderbird need to use a generated app password.

Have you logged on to webmail account via a browser and created an app generated password? https://ca.help.yahoo.com/kb/rogers/generate-manage-rd-party-passwords-sln15241.html

Check your smtp server Settings:

• Description: email address
• Server name: smtp.broadband.rogers.com
• Port: 587
• Connection Security: STARTTLS
• Authentication Method: Normal Password
• Username: full email address

Check your stored password: Does it contain the correct app generated password in both incoming and outgoing?

• 'Settings' > 'Privacy & Security'
• Scroll to 'Passwords' section
• click on 'Saved Passwords'
• click on 'Show Passwords'

If you get asked for a password in a 'Windows Security' small window - when you start up computer and logon to your computer User Account - that is the password you need to enter.

You should see two lines for the account, incoming and outgoing

• Pop uses: mailbox://account....
• Imap uses: imap://account....
• SMTP uses : smtp://account....

Check the stored password for both of the lines.

• Right click on each line and select 'Edit Password'
• completely remove contents and enter the correct app generated password.
• click on 'Close'

Exit Thunderbird, wait a few moments for background processes to complete and then restart Thunderbird.

Please report on results.

Hi Bob

What's the situation ? Is it resolved ? Did the info I supplied provide the answer ? If yes, then please mark my previous comment as 'chosen Solution'. Thanks.