What is the exact error message?

"An outgoing (SMPT) error occurred while sending email. The server responded: message rejected"

The error message is apparently from the server. You will need to ask your email provider why they rejected your messages. Only they can tell.

My ISP tells me it is a T-Bird problem that they have encountered before.

Then they can certainly tell you how they solved it the last time.

Well, except for the unfortunate fact (as I indicated) that the ISP has already told us that is a problem for T-Bird to fix and that they have had a couple of similar questions from T-Bird users today. Anyway, guess it's time to move on.

The service provider says the problem is with you. Have been using Thunderbird for as long as I can remember but can delete the program if this is all you have to offer for tech help.

Clearly the problem is not solved for any of us. Mozilla? Over to you.

The providers server issued the error message. How do you or your idiot ISP figure it is a Thunderbird problem.

So it's a Google (gmail) problem? That's the email host that is rejecting the emails.

The server rejected the message. The email provider owns and operates the server Thunderbird has no servers. Thunderbird is software running on YOUR computer Ask your provider why they are rejecting your messages.

gmail is only rejecting emails sent to an entire list on my T-bird address book. When I send to a sample of individual addresses from that same list they are not rejected. Surely that narrows it to a setting on T-bird?

Then I suggest you edit your list and clean up any errors or bad addresses.

The email list was imported from another machine, from which it worked OK. Is there possibly a setting within T-bird that affects this?

Already done this

Continue in the tread you started. This is not your thread.