As explained in a blog post on MSDN, if you use Exchange Online (or Outlook.com) you may start seeing a question mark (?) 
in place of the photo/initial on received emails.
Do not worry, this is an additional information level provided to end-user to protect themselves against spam or phishing.
Microsoft is adding this additional level of information to end-users when the email protection system has not been able to confirm for sure this is a spam/phishing or legitimate email.
This may come from old system generated email, legitimate ‘bulk’ sender (mailing list)… because it failed to be validated/authenticated (using SPF or DKIM).
For those who may be in the sender situation, it may be a good time to take a look at your systems and configuration to ensure you are not getting into the ‘unknown’ category.
All the details here https://blogs.msdn.microsoft.com/tzink/2017/09/05/showing-a-question-mark-in-the-sender-photo-when-a-message-is-not-authenticated/