Since this is only a support forum, and not a development one, I will just share observations. What you are requesting is a mailing list. Having worked with mailing lists and email clients, what you need is a real mailing list. One where I had good success is Mailchimp - and it's even free if mailing list isn't big. Short of that, you could set up a basic web host and domain for a small annual fee, as many web hosting companies offer a free mailing list product as one of their services. I seriously doubt that the decision makers for Thunderbird would even consider development of mailing list functions for a free product. But that's just my thoughts... :)

Thanks for your reply David, which helps focus the question. I no longer feel qualified to contribute to a development forum, but would like to express what, from experience with several associations and a micro-enterprise, is a frequently-occurring set of requirements that are not currently being met.

Not so long ago, the length of mailing lists was determined by the price of stationary and postage stamps. Nowadays, many more people can participate, but free and commercial IT offerings haven’t fully caught up.

Numerous voluntary (unpaid) associations participate in various ways in public life and governance. In return, we must be very careful about the rules of personal data protection, though the office work is usually done by older people like me who have the time but are not always IT-savvy. This isn’t a field for powerful business applications and services like Mailchimp.

I think the only IT dataset needed by associations like ours which don't handle much money is the address book. To maintain data integrity, there should be only one copy, and since this is used for managing mailing lists it should go into the mailing application. Thunderbird has, in fact, all the required functionality including standardised import and export formats. It’s the way this is applied at the front end for everyday use and maintenance that makes it more difficult than it needs to be for long mailing lists, and above all difficult to respect the data protection rules.

For more than a decade, I have run several personal and association accounts on the Ionos platform, each with its domain. Ionos is reputed to be an inexpensive hosting service, but it’s certainly reliable. Most often I have been using Thunderbird as client. Since the present account with the long address lists will be run by 2 or 3 people, we will use the Ionos online application Webmail Basic (we don’t want to synchronise client software).

Apart from the appearance on the screen, Webmail seems to be identical to Thunderbird, and it demonstrates exactly the same features that I think could be improved. Therefore, since I don’t know of any Ionos user forums, we may as well discuss the subject and try to define requirements somewhere on the Mozilla platform.

I understand your concern and, as I indicated, I have no decision authority here -- but my intent is to help. You will never get Thunderbird to be a meaningful mailing list. There are inexpensive tools already available that focus on your issue. Trying to twist a free email client to do that is not likely to happen. And products such as Mailchimp are free for up to a 1,000 messages a month. If you had an audience of 200 recipients and sent one message a week, you would never pay. You will also face the issue of being blacklisted for excessive mailings, and a mailing list vendor helps prevent that. I will leave you to your pursuits. I encourage you to focus your energy elsewhere for a real, workable solution that is available immediately. Best wishes. I administered mailing lists for years and learned that using the right tool always pays off.