Please provide more information, such as any error messages, plus screenshots of account email settings pane and SMTP server pane. Thank you.

Thunderbird won't open now, after more than 30 years of success. Messages say 'authentication failed'.

I have checked the manual settings and test run says all well.

It is vital that I restore the availability of this program, which is used for downloading from Xtramail to my Windows 11 PC.

Can anyone suggest what could be going wrong?

As I stated, until you share more specifics, such as screenshots of your settings, it will be difficult for anyone to suggest ideas.