So I recently installed Thunderbird (latest version) on my laptop. Most of it is working wonderfully, but, while testing, I discovered that one of my outlook addresses can only receive mail, not send. I get "Sending of the message failed. The message could not be sent using Outgoing server (SMTP) smtp-mail.outlook.com for an unknown reason. Please verify that your Outgoing server (SMTP) settings are correct and try again." Strangely, I get this message even after changing smtp settings to Server name: smtp.office365.com. I have three other hotmail/outlook account, two of which are configured with smtp-mail.outlook.com and one with smtp.office365.com. All are functioning. The other settings used are Username: ####@outlook.com; Authentication: normal pw; Security: STARTTLS. I use the same settings as in the accounts that are working. I can send and receive mail when logging on to outlook in the browser. I have tried every option I can think of. Are there any hidden settings for outgoing mail servers, in TB?, that could make the same (visible) settings work for one account, and not work in an other? I would be grateful for any suggestion. Daniel O.