Thanks for the reply, Andrew. I'll try to answer your questions as well as I can.
-I do not receive any bounce back messages. I have sent to the address using GMail, Road Runner, and even my txt service. They just seem to drop into an abyss.
-I'm not sure how to send "from" the address, exactly. When I use Outlook to compose mail, it sends from my (default) mark@ address. If I enter webmaster@ into the From field, it converts it back to mark@ for me. I can choose to send as other users whom I have permissions to send on behalf of, but since webmaster@ is an alias for my own account, this doesn't help me.
-We use a firewall to scan our incoming messages for spam, but I can see nothing caught in the spam filter for webmaster@ when I look. This is odd in itself, since that address normally receives more spam per day than any other. My default address, and a few other aliases, DO have mail caught in the filter, as they should.