There are many ways that the 'innocent' get their email address spammed to death! Mostly because they give it away freely to any website that asks for an email address in return for some freebie or other, or they publish it for all the world to see (and abuse) on their website.

Spammers do send email 'blindly' to common names like mike, john, david, sales, info, enquiries @somedomainorother.com in the hope of getting a 'hit', and there's very little we can do about that except employ anti-spam filtering in an attempt to weed out the dross, or not use 'common' names for email addresses.

Spam Filtering with Spam Assassin
Our mail system uses Spam Assassin as it's primary mail filter - This software is uncomplicated to use, and is very effective in reducing the amount of spam we get in our mailboxes.

Spam Assassin is easy to use, and has one primary setting; the spam score.
Set the score lower (around 3 - 4), and more spam is eliminated, albeit at very low settings (around 2 - 2.5) Spam Assassin can see legitimate email as spam, and dispose of it before you see it.

Adding trusted senders to your whitelist will ensure that their email will always get through.

The blacklist isn't normally much use because most spammers spoof email addresses, they are not real, so adding an address to the blacklist is pretty pointless.

Spammers regularly spoof the 'From:' address to the same email address as they are sending 'To:', and accounts for over 95% of the spam that gets past the mail filter.
If you are using server-side Spam Assassin, then by default, your email address is effectively in the approved senders list for your domain, so add your own email address(es) to the blacklist (unless you have a legitimate reason to send email addressed to yourself).

Other ways to reduce spam are...

Reply in the first instance using a 'throwaway' email alias, and only when you are happy that the person communicating isn't a spammer, then reply to them using your real email address.