Do you mean you get a delivery failure notice that tells you there are too many recipients? Thunderbird has a built-in limit of "approximately 60" addresses. http://kb.mozillazine.org/Limits_(Thunderbird)#E-mail_addresses Is it likely that you exceeded this number? This limitation can be worked around by using a Mailing List. Is also possible that the warning came from a server. Can you provide a screen shot of the error message?