Using the New Outlook for Mac for personal email and calendar
Hi. I'm Brian and I'll try to help.
If you're using a personal account and Outlook.com for the mailbox, Outlook.com has never been a good service for sending mass mail. Microsoft isn't consistent in its limits. Use a service designed for that, like mailchimp or groups.io. Your recipients can control more about which messages they wish to receive that way as well.