How can I determine if the Thunderbird profile is the source of SMTP issues and how to fix it?

To test if the profile is causing issues, create a new profile in Thunderbird by using the Profile Manager (usually accessible by running `thunderbird -p` in the command line/terminal). Configure the new profile with your email account settings. If the SMTP error disappears in the new profile, it indicates that the old profile is corrupted. You can try to repair the old profile or simply migrate your emails and settings to the new profile.