Ask HN: How often do you reply to emails/voicemail/text?

I had a discussion with an acquaintance recently. I have made a mission to reply to anybody who emails, texts, or leaves me a voice mail. My argument is that if somebody makes an effort to establish contact with me, I should at least acknowledge it.

I'm also doing this because I would hope that when I email others, they would also reply.

His argument though is that it's unreasonable for people to expect a reply. People get busy, etc.

So HNers, how often do you reply to emails, sms, etc? Do you ignore emails a lot?

My practice is that I reply to an email thread at least once. I also tend to ignore emails from headhunters especially those that looked like they were auto-generated.

Follow-up question is, would you email somebody who probably won't give you a reply?