Linux (recent) connection to smtp.office365.com now fails

[JohnFluter]

I'm sorry to reopen this issue.

I am getting the error with smtp.office365.com now. (smpt.mail.outlook.com previously)

I have a feeling that passwords and security are involved. The one time it worked was immediately after changing my settings (and entering my password).

I have deleted my password in preferences but I cannot connect now so I am not even asked for a password...

"Sending of the message failed.
An error occurred while sending mail: Outgoing server (SMTP) smtp.office365.com is unknown. The server may be incorrectly configured. Please verify that your Outgoing server (SMTP) settings are correct and try again."

The problem is not solved.

[JohnFluter]

Oops.... correction, should be (smpt.outlook.com previously) in the last post. my head is beginning to spin.

[JohnFluter]

I'm still chipping away at this problem. I managed to send one message today via smtp.office365.com before the error kicked in. Could it be anything to do with proxy settings in Thunderbird?  

[JohnFluter]

I fiddled around, set things back to normal and with no proxy im TB.

After rebooting I managed to connect and send three emails.

Closed TB. The re-opened it.

I haven't been able to send anything since!

[JohnFluter · Accepted solution]

At last I believe I have found the answer, its here...

https://ubuntuforums.org/showthread.php?t=2270096

For my TB 45.5.1 the setting is at...

Edit>Preferences>Advanced>General>Config Editor

Click 'I'll be careful'

Type ipv6 into the search box.

Double click the entry that is found, setting it to true.

Now, as a result, network.dns.disableIPv6 is disabled.

It will appear bold because it is now user set.

I will not mark the issue as solved until I have let things run for a few days but I am pretty confident!