"Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) outbound.att.net was lost in the middle of the transaction".  

I received a complex HTML encoded email. I replied to this message and forwarded it to a friend. That friend replied to that message. But when I replied AGAIN to the replied message , it failed to send, alerting me to an SMTP issue. Send Message Error "Sending of the message failed. The message could not be sent because the connection to Outgoing server (SMTP) outbound.att.net was lost in the middle of the transaction". 1. received original error 2. replied to friend. 3. received reply from friend. 4. reply back to friend failed. All other email sends without issue. What's strange about this issue is that the same content (originally received) sends ok the first time by me. Repeated attempts to send the same email fails for the same reason. I deleted the email reply and replied again and it failed multiple times. Restarted TBird. No change. Restarted Mac. No change. ATT support is worthless as they only confirm the TBird settings which have worked fine for 5 years. ATT subcontracts ATT email to yahoo, so don't get me started about the lack of ATT tech support for email they provide as part of their package. Besides, it's the ATT SMTP server communication that is failing, not the ATT mailbox processing itself so yahoo support isn't going to help either. Yes - finger pointing exercise in tech support. Bottom line, I've hoping Thunderbird users have encountered this. ATT tech support only seems to understand "the Outlook". Yes, that's the phrase ATT agents talk about email. No issues with receive side. So one would suspect some content in the email that is offensive to the SMTP service. Yet the same (complicated HTML) was in the original email before the series of replies, right? And that was replied to 2 times without issue. I use Thunderbird 45.5.1. SMTP setting is outbound.att.net, port 465, SSL. OS X 10.10.5 on a Mac Mini. ATT Uverse 10M DSL service. 100M connection to the DSL NTU. I tried to attach png capture of error message and the html archived version of the email, but adding images in the support request HTML form failed to complete upload (even for these tiny files). Tried PDF file as well. Tried multiple times yet "add images" failed to complete after 10 minutes.