Reminder: changes to how Office 365 enforces the From address will come in effect soon.

The rollout starts November 9th, to be more precise. Some legacy email servers/appliances might start having issues around that time, so make sure you are prepared for the coming changes and all messages you send are compliant with RFC 5322.

You can get all the details in this article and the references therein: https://support.office.com/en-us/article/How-Office-365-validates-the-From-address-to-prevent-phishing-eef8408b-54d3-4d7d-9cf7-ad2af10b2e0e