1) What is your domain - It could have something to do with your SPF record
2) Are you using Outlook (desktop) or online?
3) Can you try sending via a different device / network - for example if you are on wifi - can you send from your cell phone (not on wifi)
I've seen that sometimes a local IP address gets blocked and although you are sending via Office365, it still has your local IP address behind the scenes
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