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Thunderbird Sending Message Failed-Connection Lost in the Middle of the Transaction

I hadn't changed any settings, but for the last day, I get the following message when attempting to send e-mail from Mozilla Thunderbird 3.0.1:

Sending of message failed.
The message could not be sent because the connection to SMTP server smtp.att.yahoo.com was lost in the middle of the transaction. Try again or contact your network administrator.

Other details: The server is smtp.att.yahoo.com and the port is 465. Authentication is used (SSL/TLS). Secure Authentication is not used. I can send with these settings via Outlook. OS is Windows 7.

Email ProtocolsThunderbirdEmail Clients

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Jason Watkins

8/22/2022 - Mon
Jason Watkins

Have you tried the settings with secure authentication enabled?

Unfortunately I don't know of a "test the outgoing server" feature in Tbird such as Outlook has.

First disable authentication as suggested above then send yourself a test message formatted in plain text. If it works there's something you may have changed in setup.

Set Plain Text in Tools:Options:Composition:Send Options. In the Text Format panel activate the pulldown list, set to Convert the Message to Plain Text.

Firebar, if I enable secure authentication, I'm told: "An error occurred sending mail: Unable to authenticate to SMTP server smtp.att.yahoo.com. The server does not support any compatible secure authentication mechanism but you have chosen secure authentication. Try switching off secure authentication or contact your service provider."

k6xt, I get the same error message as originally posted if I use the option "Convert the Message to Plain Text". If I disable authentication (not just secure authentication, but authentication altogether), I get this message: "An error occurred while sending mail. The mail server responded:  authentication required - for help go to http://help.yahoo.com/sbc/dsl/mail/pop/pop-11.html.  Please verify that your email address is correct in your Mail preferences and try again."
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Jason Watkins

Glad to hear you have it worked out!