Google detecting webmail as malware

Recently one of our clients compained about his site being flagged by Google as "malicious" or "malware site". I added the client domain to my Google Search Console to find what the culprit was. To my surprise, Google detected my clients webmail as Malware (see attached image) That's the second time this happened to this domain. I already requested a review last week but after 3 days it happened again. What can i do about it?