This morning Thunderbird was updated to 78.4.0. Incoming mails (POP3) work fine. But suddenly outgoing mails are rejected: "An error occurred while sending mail. The mail server responded 450 4.1.8 <my email>: Sender address rejected: Domain not found. Please check the message recipient ".... recipient ..." and try again. Sending mails worked fine till update was installed, and I have not changed any SMTP settings. Same error for all recipients of mail. Any suggestions?