By the end of October, Microsoft will block three additional file types (.iso, .cab, and .jnlp) when attached to Office 365 email messages. Files of these types will be stripped from any messages delivered to the email recipient.

This step is being taken by Microsoft to further reduce incidents of malware infection. These attachment file types can be used to distribute malware via email and are rarely used as attachments to legitimate email messages.

Other file types already on the list of blocked file types include .exe, .com, and .bat files. For a full list of file types which will not be delivered as attachments to Office 365 email messages, see IT@Cornell’s Attachments Not Getting Through.

Users with a business need to share blocked file types should consider using secure channels such as web-based collaboration or file sharing services, where the sender and recipient identities are authenticated and well-known to each other.