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When accessing an email which has active content in OWA, all data may be blacked or x'd out. I would suggest adding this given email address as a safe sender and see if the issue persists.
This is unfortunately a normal security precaution that OWA takes against active content as David said and not specific to Office 365. The most simple way around this is actually to doubleclick on the message. You should find that when the message is opened in a new window and you have clicked on Yes when you are prompted to display the nonsecure items on the page that you will be able to read it.
If the proposed response resolves your question please mark this question as answered so others may benefit from the suggested resolution.
I just tried adding both the sending address and the sending domain to the safe senders list, however the message remained blacked out. Once I double clicked on it to open in a new window it displayed fine.
That seem a very odd 'security feature', if it can be bypassed just by opening the message. Hopefully there will be a proper option in the future to enable active content from safe senders.
Seems odd as O365 sends emails with active content by default for new users, but one display them properly. I can see that being very confusing for newusers.