Based on their limited instructions, I would try these settings:

outgoing server smtp.slingshot.co.nz. port 587, STARTTLS security, authentication = normal password, User Name = part of address before the @. Open Account Settings, then Outgoing Server (SMTP) at the bottom of the left pane.

Thanks for your reply, I checked those settings earlier when I looked at the similar problem - that advice was the same as yours, which is how my account settings are set up and have been. So I'm afraid that hasn't helped, but again - thank you.

Do you use a VPN? That could cause an authentication error. Any recent changes or updates to your antivirus?

Running Windows in safe mode would test the effect of either of those.

Sorry, this isn't working either. I'm not sure where to go from here, can I reload the previous level..?

I think you have to contact the mail provider, as an authentication error indicates either the password, authentication method, or some other server setting is wrong. Downgrading TB to an earlier version is not recommended and anyway seldom fixes issues like this. Try adding the account to the Windows mail app and see if the same errors occur.

Thanks for your help and advice. I have been to and fro with slingshot, tried their suggestions which are to send emails from my slingshot account, which works OK.

Therefore their position is that the problem is with Thunderbird, their last suggestion is that I delete Thunderbird and reload, which may make sense, except that I don't want to lose the information I have there.

I don't have Windows mail app., only use Laptop for emails. Oddly though, my wife's laptop has the same level of Thunderbird, and same account settings and has no problem sending emails - so definitely something with mine.

Uninstalling/reinstalling TB seldom fixes problems with the (separate) profile folder. But testing with a new profile might help: Help/Troubleshooting Info, about:profiles.

Windows 10 & 11 have a built-in mail app now called Outlook, that includes mail, calendar and contacts.

Maybe your PC has an external app like antivirus that's different from your wife's. As mentioned above, run Windows in safe mode to test the effect of startup apps like AV that are bypassed in safe mode. AVG/Avast are common causes of sending errors.

Hello again, Many thanks for your help and advice, and my apologies for not coming back to you sooner. I'm working slowly through this because of my low tech savvy., but am learning a lot as I review what and how I go about a 'safe mode' start. I need to be in the right head space (because of illness) to do this. I will come back to you once sorted.. :)

Hello again, I still have the same problem with my email account, however, I have worked around it by setting up another email I.D. which works fine. So I can receive and send emails OK now, so maybe called the problem solved..? Thanks again for all your advice, help and patience..! Roger