remove dangerous attachments

The “remove dangerous attachments” option blocks all potentially harmful attachments
except some safe extensions as pdf, txt, gif, jpg and png.

The recipient receives the message without the attachment.
A warning is added to the beginning of the content, like this:

WARNING: This email violated Your Company's email security policy and
has been modified. For more information, contact your IT Administrator.

An attachment named "example.zip" was removed from this document as it
constituted a security hazard. If you require this document, please contact
the sender and arrange an alternate means of receiving it.

On the internet there is an interesting case study, that ends with this sentence:
“For us, attachment filtering has been very successful”
– web.mit.edu/net-security/Camp/2004/presentations/reillyb-mit2004.ppt (PowerPoint presentation)