When I try to send an email to 2 email groups, adding the second group displays "Warning: you should move these recipients in bcc" and I can't send it.
When entering the second emailing list, I click on the auto-fill for the name of the list. Rather than adding the email addresses, I get a big yellow bar overlapping the TO: field with the following message:
"Warning: you should move these recipients in bcc"
After than, I can no longer perform any actions in the To: field and cannot send the email.
Alle svar (1)
I have never seen nor heard of Thunderbird itself displaying this message. Maybe you have an add-on? There is one called Use Bcc Instead which reminds the user to use Bcc, but has never actually prevented me from doing what it warns about.
I'd suggest you review the add-ons installed in Thunderbird, under Tools|Add-ons in the main menu.
And maybe you could provide a screenshot of the warning message. It may remind someone of an add-on they use or have used.