Attachment blocking prevents attachments in email messages from being delivered to the Microsoft Exchange Information Store. Configure the Messaging Security Agent to block attachments according to the attachment type or attachment name and then replace, quarantine, or delete all the messages that have attachments that match the criteria.
Blocking can occur during Real-time, Manual, and Scheduled Scanning, but the delete and quarantine actions are not available for Manual and Scheduled Scans.
The extension of an attachment identifies the file type, for example .txt, .exe, or .dll. However, the Messaging Security Agent examines the file header rather than the file name to ascertain the actual file type. Many virus/malware are closely associated with certain types of files. By configuring the Messaging Security Agent to block according to file type, you can decrease the security risk to your Microsoft Exchange servers from those types of files. Similarly, specific attacks are often associated with a specific file name.
Using blocking is an effective way to control virus outbreaks. You can temporarily quarantine all high-risk file types or those with a specific name associated with a known virus/malware. Later, when you have more time, you can examine the quarantine folder and take action against infected files.