Sending from a Thunderbird-hosted 365 account generates Windows Security popup

Hi all,

I have several email accounts, one of which is hosted via Microsoft 365. Every time I send an email from that account, Windows Security opens up a dialogue box, asking me to select a certificate for authentication onto smtp.office365.com. It offers a Microsoft Your Phone certificate (see image attached),which works OK, but being Windows, the dialogue box opens behind the other open apps without any further notifcation - and if I'm busy, I won't notice it, and the email won't be sent.

I've tried installing the certificate, but that hasn't changed anything.

I've also attached a screenshot of my SMTP settings.

Can anyone suggest a permanent solution?

Thanks in advance.