Bah!
A properly configured antispam solution will handle nearly all inbound spam. That should take place at the gateway/perimeter. Any legitimate business needs to address it. It doesn't matter if the user advertised their email address on a billboard in the middle of town, or the user never gives out their address. A business email system will be sent spam. Period.
Imagine this - you have a user who sends their spouse an email. The spouse saves the email address of your user in their Contacts/Address Book. Their machine gets infected with an email-bourne malware that grabs the Contact list and sends it back to a database. Neither user did anything wrong - yet now the email address is on a list that will be sold to spammers. You simply cannot stop an email address from getting publicized. You can only deal with the results of that.
And a business has no way of knowing HOW that address got onto a specific spammer list that resulted in a specific email getting sent to their mailbox.
Imagine a user goes on vacation, and turns on their Out Of Office setting, with a message that says "I'm away.... for assistance, please contact bob@yourdomain.com". Spam hits the mailbox, gets the OOF reply, and now knows two things: the original email address is valid, and so is Bob's.
Any business that says it's the users fault for getting spam is just trying to deflect the blame to avoid having to admit fault. There are many simple solutions that resolve the problem nearly 100%. Some are cloud based, some are on-premise. But all should happen near the perimeter.
I don't say this because corporate messaging is my line of work and I want some business. I say it because I see this all the time, it's not terribly difficult to resolve, and a businessed can reap the rewards by addressing it with determination.
Pat Richard MVP
Plan for performance, and capacity takes care of itself. Plan for capacity, and suffer poor performance.