When configuring outgoing mail using office365 SMPT server, test email gives error

[Iana]

Hi,
We have an outgoing mail configured to automatically send notifications regarding our tickets.
Since tuesday Jan 24 no notifications have been sent.
I tested the outgoing mail configuration and the connection is succesfull (see SS attached).

But when I try to send a test email, I get an error

MTPSendFailedException: 451 5.7.3 STARTTLS is required to send mail [LO4P265CA0197.GBRP265.PROD.OUTLOOK.COM 2023-01-30

I found a community post that suggested ticking the TLS option and tried to apply it, but when I authorize the new configuration, the button automatically unticks.

Do you have any ideas what could be going wrong?