We all ignore some emails. It’s almost culturally acceptable, particularly if the email is out of the blue and you have no ties to the person or company they are with.
Hey I’ve got a thing! You should see it! Let’s set up a call!Person you don’t know
It certainly feels more polite to respond. Sorry, sir, I don’t have time for whatever it is you want. (“Don’t have time” being the culturally acceptable wording of “I don’t care about this.”) Even if the out-of-the-blue email isn’t polite in itself, part of the spirit of being polite is being polite no matter what.
The danger is that, in responding, you’re opening up the door just a little bit. Like making eye contact with someone belligerently drunk. They might see it as in invitation to engage.
Tough call, really. For me, it sort of depends on what kind of mood I’m in.