Hi All, I just added a second Office 365 account to my Thunderbird Profile. That broke the ability to send messages from either account. Thunderbird never sends an SMTP response after the server replies. I removed the second account, deleted both SMTP servers and then restarted Thunderbird and I was able to send again from the first account. I added the second account back, and once again, neither account is able to send. This time, I left the first account at smtp.office365.com and the second one at what it auto-discovered, smtp-mail.outlook.com. I tested this with 52.5.0 and with 58.0b2 (64-bit). I think this is a bug of some kind, but maybe I'm missing something. Attached are a screenshot of the mail settings, and the packet capture. Cheers,