Hey there!
I'm sorry that you are facing this issue.
It looks like Outlook.com's anti-spam safeguards are being triggered, but only for certain recipients or group messages
Few tips you can to see if it helps.
Send a test email to each person in the group individually to identify which address may trigger the block.
If you're using a contact group, recreate it manually . One of the contacts may have a corrupted or outdated address.
Check for any links or attachments in the message that could resemble spam triggers.
Wait a few hours and try sending it again. The temporary flag often lifts quickly.