Office 365 (Known Issues) - AdBlock Browser Add-on Rejecting Messages in Safari
Individuals report they do not receive emails and/or receive a failed delivery report after sending an email. This problem is related to a bug in the Safari web browser add-on called "AdBlock". AdBlock appends additional code to Office 365 emails, causing many spam filters to flag messages as spam. This is not an Office 365 bug.
Resolution Status: Unresolved. AdBlock is aware of the issue and working on implementing a fix. Please see their support site for more information.
Disable the AdBlock browser add-on, use a different web browser, or configure Mac OS X Mail to connect to Office 365.