If you are not able to open the email attachment that is sent via Optimum Mail then you should try resolving this issue on your own. And for that, you have to just delete the footprints of your regular browser which can be done when you remove all the cookies, cache and history data. You can also try to use a different browser for opening the attachment if it again failed to open then you can just reach Optimum Customer Support and try to speak with professionals.