problem sending mail to microsoft recipients...

Hi, we have a mailserver having problems to send mail to microsoft mail addresses (hotmail, outlook, 365...) Our config did not change and looks okay. Anyone who can see what could be the solution to fix this?
This is a part of the error msg:
This is the mail system at host mail.mx10.nl.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

: host mail.protection.outlook.com
said: 451 4.4.4 Temporary server error. Please try again later ATTR3
[HE1EUR01FT050.eop-EUR01.prod.protection.outlook.com] (in reply to MAIL
FROM command)

said:
451 4.4.4 Temporary server error. Please try again later ATTR3
[HE1EUR01FT050.eop-EUR01.prod.protection.outlook.com] (in reply to MAIL
FROM command)