One mail to many receivers

Sorry, but that is the wrong question. The question you need to ask is "How many receivers can you send one e-mail by " enter name of your mail provider here. Thunderbird has no real limit, although I would guess there is a technical one somewhere in the tens of thousands. I say a technical limit, because all mail providers I have ever used or encountered limit the number of recipient to a message and that limit is well below what Thunderbird will llow you to add to the To: CC: and BCC: fields. What the limits are depends entirely on the provider. Gmail for instance has a limit that they define as "you send an email to a total of more than 500 recipients in a single email and or more than 500 emails in a day sent."