JeffC

Reinstalled Premium and amended the setting you suggested and emails now being sent successfully. Thanks harlan4096?

Thanks for the speedy response harlan4096. I assume you mean Settings in the Kaspersky program, in which case I cannot see Security Settings within Settings. Of course I am using Total Security now as I backed out Premium, so I cannot see what options may be available in Premium. If I reinstall Premium and the fix you suggest doesn't work then I have a fair amount of activity to get back to a point where emails can be sent again ??

I installed Kaspersky Premium yesterday (11 May 2023) but found that I cannot send emails via Mozilla Thunderbird client. Premium was the only configuration change made to my Windows 11 PC so I suspected it to be the cause. Error was "Sending of the message failed. The message could not be sent because the connection to outgoing server(SMTP) smtp.office365.com was lost in the middle of the transaction. Try again. I stopped Premium, uninstalled it and downloaded my old AV (Kaspersky Total Security) but this is due to expire in 1 DAY!! Emails now being sent OK. I could not find a reported error with Premium/Thunderbird so I am posting on here in the hope the issue can be quickly resolved. Any assistance asap will be appreciated.