Sending, receiving, and organizing email in Outlook.com
Hi, Microsoft 365 increases your Outlook.com sending limits, but it doesn’t remove them. Microsoft lists these limits for subscribers:
500 recipients per message
5,000 recipients per day
1,000 recipients per day whom you’ve never emailed before
Limits can also be lower depending on your account’s usage history.
https://support.microsoft.com/en-us/outlook/sending-limits-in-outlook-com
How many recipients are you trying to send to, and what exact error message appears? Please remove any email addresses if sharing a screenshot.