Error 5.1.3 "incorrect character "@"
Cannot send a group email to contacts because of the above error. How can Thunderbird possibly object to the "at" sign = @?????
Wšykne wótegrona (1)
Thunderbird doesn't reject this, but you mail provider does. Error 5.1.3 is a server error. With the information you sent however, it is impossible to say why the message is rejected by your mail provider. I suggest that you check all the addresses in the recipients lists. If you can't find the error, maybe you can post the message here, but remember that it will be visible to the public if you do so.