They are doing the same thing you are doing, but in a way that might not get it delivered to some of your recipients. The "undisclosed recipients" is put on by GMail to indicate there was NO "to" line. Some companies automatically mark this as spam since it has no "to" line. Your best bet is to include *1* e-mail on the "to" line.
If you're not including a "to" line, then you are already doing exactly what they are doing; but as I said, some of the people you're sending to may never see it if their mail carrier classifies it as spam before they even know about it.
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Isaac Asimov - "The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'E...