When replying to another user with the @ symbol, (e.g., @matt), closing up the space between the symbol and the handle will automatically link to that handle and may generate an email message for that user. (depending on their preferences).
If you insert a space, the forum software appears not to link to the handle. (e.g., “@ matt”). If you are replying to John Q. Public (@JohnQPublic), and you want to invoke his handle, you need to close the space and type the full handle. If you do not want to do so, I think you can say “@ John” or “@ JohnQPublic”.
Note that should you write “@john”, however, then some other John whose handle is just “@john” is going to get all your messages. I know this because I’m that guy. I like you all, but you might want to talk to some other Matt on occasion.