This has been asked a few times recently, see this question. I think it is just a temporary server issue allright, as the message says. For outlook issues like these, you should also look here first. You can click the map to see issues in your area.

It may be safe to set up another SMTP server in Thunderbird, which could also be the one from your own provider (ISP). If you add an identity in Thunderbird for the Outlook account, you can set it up the same way as the existing one, except for the SMTP server in the drop-down that you can change there (instead of editing the current SMTP server that may be the only one set), so you can select it when composing a message when the issue occurs.

There is a small chance however that either your own ISP’s or a remote server will reject sending/receiving if a sender’s email adress does not match the domain.

Does this help?

Thank you for your help. So to sum up, I am no longer able to use the original smtp server that used to work up until about a week ago: smtp-mail.outlook.com. And the one Microsoft suggested I try , smtp.office365.com, does not work either.

So I had to Add a new Outgoing Server (smtp) for my own isp's smtp server and I went ahead and edited my existing Identity to use the new Outgoing Server (smtp) as well. So I have imap from outlook.com, but smtp from my ISP. I happen to use an email account name which corresponds to my ISP, so I think that helps make this configuration successful. I had to also change my Thunderbird Account Settings under "Copies & Folders" to save a copy of the sent message in my Sent folder. Previously, when using the outlook.com smtp, that happened automatically on the outlook.com side.

Good to hear and well done, thanks for your feedback.

There may be a small glitch: instead of editing the existing identity, my intension would be to add a second one for the Outlook account (when the account name is selected on the left, above Server Settings) and apply all changes compared to the original setup only there, while leaving everything else untouched. That’s the "power" of identities.

You were right about the Copies & Folders settings - I forgot to mention those are important for the saved copy happening remotely when using the Outlook SMTP server, but you could set that up in its own tab for the identity you added instead of the general settings for the account, similar to the chosen SMTP server.

If you set things up this way, all original settings remain unchanged, and the only thing you would need to do when sending a message using the Outlook SMTP server fails, is change the From: field during composition to use the non-bold sender (=other than default identity). And whenever the issue is long-standing, you could simply set the second identity as default identity for how long it lasts to prevent choosing the other "From" every time, and switch back when it’s solved.

Feel free however to leave things as they are. :)