Hi Jonathan,
Sorry for the inconvenience.
I understand that you encounter the specific issue when sending mails from Mac Mail using an Office 365 mailbox. Currently, Mac Mail clients on the Mac OS version 10.8 or higher is not supported in our forum. If you’re using Mac OS 10.7 or earlier, please refer to: https://support.office.com/en-us/article/Set-up-email-in-Mac-OS-X-Mail-de372dc4-9648-4044-a76c-e8a60e178d54?CTT=1&CorrelationId=fb1b5155-bda2-41cb-ae9a-ff9f5f9c8995&ui=en-US&rs=en-US&ad=US to set up the Office 365 mailbox on it and check if it works.
As a workaround, I suggest you use other supported mail clients like Outlook for Mac for Office 365 to manage the Office 365 mailbox on the Mac. Your understanding is appreciated.