If this were even a tiny bit of an issue then way more spam would hit people's inboxes. Rejecting explicitly is the right thing to do. It's not always possible,

If this were even a tiny bit of an issue then way more spam would hit people's inboxes. Rejecting explicitly is the right thing to do. It's not always possible, but it's definitely preferred. Source: I wrote some of the original SpamAssassin and worked in anti-spam for over 10 years.