Bcc: is designed for this.

If you address a message with To: or Cc: being used, these will be visible to all recipients.

If you include addressees with Bcc: then, in general, these recipients will see the To: and Cc: addresses, but not the Bcc: addresses.

Note that this behaviour isn't guaranteed; it is possible to interpret the specifications to mean that Bcc:'d addressees may see the other Bcc:'d addresses. You need to check how the smtp servers you use treat this.

If you use a Mailing List and add it to a Bcc: addressing box, then all of its member Contacts will be Bcc:'d.

If you don't have any To: or Cc: addresses, then Thunderbird will send the message with "Undisclosed recipients" in the To: field because a few smtp servers will refuse a message without a visible To: address.

We converted from Eudora to Thunderbird some time ago, and we have not tried to send a message out to our imported mailing list recipients. The reason we have not done so is we cannot find a way to suppress the addresses. And, we definitely do not want all of the recipients addresses being shown to each other. Bad internet etiquette.

Not being able to do this in Thunderbird is a big disappointment. It was so easy to do in Eudora, and it was 100% reliable.

If anyone finds a way to do this, please let me know.

How did the advice above not apply to your problem?

I suppose I need a better explanation of this:

If you use a Mailing List and add it to a Bcc: addressing box, then all of its member Contacts will be Bcc:'d.

OK. You are correct. When you create the new message, below From: change To: to BCC: before selecting the mailing list name (I just start typing the name of the saved list).

I just tested this and it works. Thanks!

OK, so I used Thunderbird to send email to a list using bcc: with the list name. The result was that the receivers saw bcc: Undisclosed Recipients, then cc: every name on the list.

Not exactly what I expected or wanted.

My mail provider is gmail. Is this an artifact of gmail's behavior rather than Thunderbird's? If someone out there knows how to fix it so the recipients don't get to see each other's names and email addresses, I'd be grateful.