Thunderbird stopped sending mail, error message reads an error occurred while sending outgoing server (SMTP) smtp-mail.outlook.com is unknown etc
Started sending intermittently for a few days now not sending at all, have to go to webmail to send. Have checked the settings repeatedly to no effect. OS is Linux MX-14
Chosen solution
Do things work when you toggle network.dns.disableIPv6 to true in about:config (in the Preferences dialog > Advanced panel > General tab)?
You may also be able to use the network.dns.ipv4OnlyDomains preference to work around it. The real question however would be what causes the DNS lookups to fail, if they do. Perhaps this page is of help for that. Or was anything related to networking changed on your machine recently?
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Any difference if you use smtp.office365.com on port 587, STARTTLS security?
No difference I am afraid.
Chosen Solution
Do things work when you toggle network.dns.disableIPv6 to true in about:config (in the Preferences dialog > Advanced panel > General tab)?
You may also be able to use the network.dns.ipv4OnlyDomains preference to work around it. The real question however would be what causes the DNS lookups to fail, if they do. Perhaps this page is of help for that. Or was anything related to networking changed on your machine recently?
In addition to the suggestions of Tonnes, I would do some simple things like the Linux-equivalent of the Windows flushdns command, and reboot the modem/router.
I think I have found the problem, it appears not to be a Thunderbird problem but a router one. I connected an old modem/router that I had replaced for a different reason and it sends mail normally without errors so thanks for the help with this.
It looks like I was wrong, the same problem has returned with either modem/router now and stopped sending completely.