Hello, I have the same problem since beginning of august. The problem occurs only if I use outlook address with older domain name@windowslive.com and try to send message using Thunderbird. E-mails never reach destination and I get this error message: ‘Delivery has failed to these recipients or groups: name (e-mail address) The mail client submitted this message with bare line feeds, which cannot be sent via SMTP protocol DATA command and receiving system does not support BDAT. Diagnostic information for administrators: Generating server: PR3P192MB0862.EURP192.PROD.OUTLOOK.COM e-mail address Remote Server returned '550 5.6.11 SMTPSEND.BareLinefeedsAreIllegal; message contains bare linefeeds, which cannot be sent via DATA and receiving system does not support BDAT' Original message headers: it contihues…’ Funny moment is e-mail sending works if use ordinary win11 mail client Any other e-mails (domains yahoo, gmx, gmail etc.) using Thunderbird do not generate this error I think there is some issue in Thunderbird that e-mails with domain @windowslive.com do not work Can someone clarify and suggest solution? Win11 e-mail client works ok, not Thunderbird.