I have had no problems sending new messages or replying to old ones on Thunderbird until this morning. My emails go through Thunderbird to an Office 365 server, and I am getting this message back from that server: "Your message contains invalid characters (bare line feed characters). This error occurs when the email program or device used to create or send an email message adds bare line feed characters into the message. When bare line feed characters are included in a message, the SMTP protocol chunking feature is required to transmit the message between email servers. Chunking uses the SMTP protocol BDAT command, but the recipient's email server doesn't support the BDAT command. To fix the issue, the sender should send the message using an email program or device that doesn't add bare line feed characters to messages. Most modern email programs don't add bare line feed characters." This problem seems to affect one mail box only in Thunderbird on one computer. I have Thunderbird on two PCs (one Windows 7, one Windows 10), and each has three mail boxes. There doesn't seem to be any problem on the Windows 10 machine (emails go through just fine), and I am able to send emails from two of the three accounts on the Windows 7 machine. I have rebooted but that didn't help. Does anyone know what could be causing this problem and what I should do? Many thanks!