I didn't expect this to be such a difficult question. Are MS 365 calendars so rare among thunderbird users?

MS calendars are only supported with the TbSync/Provider for Exchange add-ons, and only for TB 140 ESR. Exchange support is built into TB 145, but currently only for mail, with calendar and contacts support in development.

Is sending emails supported? Receiving them works fine but sending doesn't. It was like that in previous versions too but I thought this was fixed in 145 since support for exchange was mentioned in the changelog.

Sending should work, but you would have to state your smtp settings or any error messages. Maybe smtp has to be enabled in O365 settings.

On my main installations I'm using Owl which uses OWA (which I believe is what thunderbird also uses) and SMTP so I know it works with my account. I don't know what server settings Owl uses for outgoing mail because this is not shown anywhere.

On my test installation without Owl I have: smtp.office365.com 587 STARTTLS OATH2

It tries to send the email and fails with a message saying: Login to server smtp.office365.com with username XXX failed. The password is 100% correct. It's the same for the incoming server which works well and in any case it's input from a password manager so typing is not involved.

Did you add the account according to the instructions here?

https://support.mozilla.org/en-US/kb/thunderbird-and-exchange#w_limitations

These instructions are for automatic configuration which doesn't work for me. My email address has a company domain but it's hosted on microsoft's servers and not on the email's domain. So thunderbird can't find the settings automatically. I have to enter them manually.

This time I tried these settings. The only difference is the SMTP address. https://support.microsoft.com/en-us/office/pop-imap-and-smtp-settings-for-outlook-com-d088b986-291d-42b8-9564-9c414e2aa040

It seems to connect just fine, downloads the server certificate and says it's valid. Incoming email works as before but outgoing still gives an authentication error.

I never get the microsoft window shown on step 3 to enter my password. I have to enter it in thunderbird otherwise I can't continue. So I never see the dialog in step 4 either.