When I try to send emails from a new email address, Thunderbird cannot write a copy to sent folder. Initially Ialso had this problem sending to the new account from an att.net address. The new email address is hosted where I have several other accounts and the set up is the same. Checked, and no problem on server end. Attaching a screen shot of message I get - and this stays indefinitely. I'm on Windows 10 Pro 64 bit. Thank you.