SMTP authentication for Office 365 - to sent e-mails to candidates is not working anymore

We have a problem with sending emails from personio to candidates. I get the following error:
Expected response code 250 but got code "451", with message "451 4.4.4 Temporary server error. Please try again later ATTR5 [VI1EUR02FT046.eop-EUR02.prod.protection.outlook.com] "

We use one of our own emailadresses to receive emails from candidates and to sent emails to.
We do receive the emails from candidates, but we can not sent the emails to our candidates.

I had contact with my IT admin. They have done the following:
My IT admin did change our hostsettings, portal and encryption for the SMTP settings. But we still get the response code. They informed me that the problem is within the software and has probably to do with the region code.

They also informed me that they could get the settings right if the connection could be made through MS Graph or OAuth2. The SMTP connection will not work for Office 365 settings anymore. This is because of a change in de Office 365 policy (which they have informed about a couple of years ago).

I would like to know if you're working on a solution. Or if you have a solution, so we can use Personio again.