TB defaults to sending a courtesy reminder that sending to more than 15 may not be well received, as it exposes email ids that, in many cases, are preferred to remain private. But it's just that, a reminder. If you prefer to stop the reminder, follow these steps:

The default for this nag is 15 recipients. You can change that default by going to config editor tools>preferences>general, scroll to bottom and click config editor and entering mail.compose.warn_public_recipients.threshold change to a number higher than you use, e.g. 99 and that should stop the nag.

Hello, thank you for your answer and help. I have changed the value to 50. However, this does not fix the issue. An error message is flashed, and cannot be read. I suppose there is something wrong in the recipient list. Also, it is not easy to test... Thanks and regards, Gilles V;

My suggestion: mark this thread closed and then open a new topic about error message flashing too quick to read. That may alert some experts on this forum on tracking log info to show explicit error. I don't know how to do that yet.

Hello, my Thunderbird 91.6.1 (64 bits) has been updated to Thunderbird 91.6.2 (64 bits) yesterday. This morning i had to send a reply to 2 persons, with a copy to 22, and i have no longer this problem... As already told I changed the 15 value to 50 so i did not get the courtesy reminder that sending to more than xx may not be well received. I do not know if the flashing error message is related to that. Anyway, TB is back to normal for me. Gilles V.