It probably blocks any attachments it doesn't recognize. My email account here blocks all attachments that are not .zips I think.
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When a cat is dropped, it always lands on its feet, and when toast is dropped, it always lands with the buttered side facing down. I propose to strap buttered toast to the back of a cat. The two will hover, spinning inches above the ground. With a giant buttered cat array, a high-speed monorail could easily link New York with Chicago.
That's not it. It claims to block executables, even if they are zipped. My tests have shown otherwise--a renamed executable (I changed the extension to stufft) makes it through unfiltered.
Also, the ZIPPED pbw file was blocked but when I sent the .gam file itself without zipping, it went through.
Thus, it apparently does not filter on "unknown extension" ...