It's just occurred to me that your problem may be in configuration of your server.
Most small business and personal hosting packages are setup by default with a "catchall" account, so it doesn't matter what people type before the @ everything for you domain is dumped in a single account.
However, there is usually an option in your hosting control panel to set up individual accounts.
e.g When my personal domain was set up, anyone could e-mail me at
[email protected],
[email protected], and so on.
When my wife finally decided to move into the 21st century, I went to my control panel and set up 2 accounts,
[email protected],
[email protected].
At that point I was given a choice of what to do with any other mail that arrived: re-route it to one of the accounts, bounce it, or junk it. As I am awash with spam, I chose to junk it.
Once you have set up individual mail accounts on the server, it's trivial to set each machine to collect mail only from the appropriate account.