Can't get rid of ads1.msads.net

Hello,

Every time I log into Hotmail (outlook.com) and every time I refresh within the site, I get a notification continue to or cancel to website ads.msads.net. I contacted Outlook and they said because it doesn't happen when I'm on a PC, it has to be a Mac problem. I've read other threads on this and the problem seemed to be the Mail Tab Pro app but I don't have it so I'm lost. Any suggestions on how I can stop this from happening? Thank you!