This is to counter spam and malware, done the Microsoft way (like Outlook not being able to receive attachments with certain filename extensions): "Michael Ata

This is to counter spam and malware, done the Microsoft way (like Outlook not being able to receive attachments with certain filename extensions): "Michael Atalla, Director of product management for Exchange Server and Exchange Online, confirmed the limitation [...], arguing that it’s a necessary anti-spam measure: 'Every online service provider must limit and constrain its service based on limitations such as the amount of disk space currently in its datacenters or bandwidth currently available and also enforce behavioral thresholds which prevent inappropriate use of the service by malicious users or criminals. This is true for any form of web-based service. In the world of email, one of the thresholds that must be enforced is the amount of email that is sent through the system by any one user or organization in order to combat spam, mass-mailing worms & viruses.'"