Thunderbird being used as IMAP client for Gmail - with no issues, until recently when an existing email recipient, who sits behind the Barracuda Networks system, fails to receive mail from Thunderbird's recent versions because it is rejected and blocked by Barracuda. I have tried using plain text formats and checked that SMTP server accounts are correct all to no avail, a 550 blocked is the result. The Thunderbird ver is 143 64 bit - However mail from an earlier 32 bit version 115.5 was successfully delivered, later 32 bit ver 140 failed to deliver through the Barracuda system.

Email from Google's own Web client gets delivered and mail from the Android mobile phone app likewise arrives. A simpler IMAP client (MailSpring) manages to get mail thru Barracuda so what has Thunderbird done to appear a threat to Barracuda ? BTW Mailspring is sort of 'Ok' - but it just isn't Thunderbird - it was installed only as a test vehicle !