I have the same problem on my Samsung galaxy a5 worked perfect up to yesterday but now unable to forward any emails. Apart from that working fine. Keep getting message loading emails with loading circle but it never loads.
Having the same issue here with several users (multiple carriers) in our corporate environment. These are Samsung Note 4 users (v6.01) using the native mail client connected to Office 365 (ActiveSync). We're guessing there was a recent update that introduced a bug into the mail client, but we haven't confirmed that yet. Only our users that are unrestricted have this issue. Our restricted users are unable to update their devices on their own and are not experiencing the problem.