I've tried everything suggested on the forums (such as below) but nothing works

https://support.mozilla.org/en-US/kb/cannot-send-messages

The admin has allowed/authorised pop/imap.

My other colleagues have the same probem. please help.

Which password are you using with Thunderbird, your primary Microsoft account password or an app-specific password generated from your account's security settings?

Hi Stans.

I'm using the primary Microsoft password. should I be using an app-specific password?

I checked the office portal and it says the account MULTI-FACTOR AUTHENTICATION STATUS is Disabled.

I checked the office portal and it says the account MULTI-FACTOR AUTHENTICATION STATUS is Disabled.

In that case, your primary password should work just fine. In Thunderbird, check SMTP server settings and make sure the UserName field contains your full email address. Also, remove the password used by the SMTP server in Tbird's passwords vault so that it prompts you for it. Make sure to check the box to save the password and confirm that it is actually saved in the vault.

Do you have other accounts on Tbird and do you experience sending issues with them or just this one account? Also, are you using a custom/private domain connected to your Outlook.com account and hosted by GoDaddy?

Thanks for the reply Stans.

I have tried all of the above. I've even tried uninstalling, reinstalling etc. We have four email Microsoft365 mailboxes that work perfectly in outlook but none will send in Thunderbird. I have used TB before and my colleages (who prefer TB to Outlook have used it for years) but since recently migrating servers from an Exchange2013 server to the Microsoft365 cloud server we can not send emails in TB.

Yes, we are using a private domain. I have access to the domain DNS or Office Portal if that helps? Must have read 50 articles but can't find anything that works.

I would have the admin get in touch with Microsoft Support about this, find out what has changed as regards using Thunderbird to access your Microsoft 365 emails and how come Thunderbird is not working with the same settings that Outlook is using without issues. I'm not experienced with the Microsoft (formerly Office) 365 platform, so there could be something that needs to be changed or enabled from the Microsoft Admin portal for non-Microsoft clients to work with that account. All I can think of is access to IMAP and/or POP, which has already been enabled and are working just fine, only that those two have nothing to do with sending mails.

Thanks Stans for your help. I did get in touch with support and they advised trying one of the Add-ons. This one worked: ExQuilla for Exchange Addon

You're welcome. Good to know you found a working solution. Further searching reveals that Microsoft plans to end support for basic authentication on their Exchange Web Services protocol. Hopefully, the add-on author will keep up and add OAuth2 support before that time comes, if it doesn't already have it.

Also, see if SMTP AUTH is enabled for your Microsoft 365 account as describe here https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission. This is needed for basic authentication to work with "less secure" apps like Thunderbird. It's possible that new Microsoft 365 accounts are setup with SMTP AUTH globally disabled by default.