I see the following line in the header that you posted earlier:

Received: from pgquv (222.67.186.87) by HK2PR0401CA0004.apcprd04.prod.outlook.com (2603:1096:202:2::14) with Microsoft SMTP Server

This means they are an email client on their desktop rather than Hotmail's web interface. Look at the sender's IP address in these emails. If they are coming from a handful of IP addresses, you can create a Header filter to look for them.