WE had this problem as well, when we sent a report via email and the receipient used Outlook.
Unless the email was flagged as allowed, it went to the 'Junk' mail folder. The person tried to view it from there, and all HTML features were turned off.
Just to be sure, we created a rule to send all email from SolarWinds to a specified folder. Also we ensured all users who received the report had HTML as the default message format.
This post might be a little late, but may be other users can bennifit from this.