From kaspersky.com
Modern antiphishing and antispam solutions increasingly draw on a variety of machine-learning technologies. Using neural networks to analyze text makes them hard to fool, so attackers have turned to a simple but effective trick: placing the text in a picture. They then embed the image in a message body using Base64 encoding (typically, images in e-mail messages are hosted on an external website, and mail clients don’t load pictures in letters from outside the company). Most such letters are after users’ Microsoft Office 365 credentials.