Spaces/Tabs replaced with Question Marks in some incoming emails when moved to SharePoint Online

Has anyone ever encountered this issue? 

We have a user who received an email in Outlook as follows,

"Dear XXXXX, 
                      I am writing today in regard to..."

However, when the email was moved to SharePoint and viewed in the Previewer, it read as, 
"Dear XXXXX, 
????????????? I am writing today in regard to..."

The user has moved multiple emails across to SharePoint from multiple sources including this one, and this is the first time they've noticed/encountered this. 

I've looked up other articles that have recommended changing Encoding settings in Outlook but I'm not in a rush to do this unless I'm convinced that it is the issue, which I'm not based on the amount of emails it has effected to date. 

If anyone has more information on this I'd be grateful to hear it. 

Kind regards, 

Chris