I maintain about 20 servers and of course one problem, irritating but not huge, is spam. About 95 percent of the spam I receive is addressed to nonexistent addresses (for example "firstname.lastname@example.org" or "email@example.com," etc.).
What I have found that works pretty well to get rid of most of it is this:
Set up necessary email accounts, then set up a "catchall" address (forwards all email that is sent to a recipient name that doesn't exist to an alternate destination). I call my catchall "junk" (full name "Junk Mail") and give it a 1MB quota. Then just leave it alone. When the "junk" mailbox fills up, the mail gets bounced to the sender with a message such as"Junk Mail mailbox full."
"Whatever can be said can be said clearly." (Ludwig Wittgenstein)