You should always insist

I have a few friends that are looking for jobs right now. I've told them when they send out their resume, they should always follow up. And so should you.
If you shoot out an e-mail and you get no response, you should always send a follow-up mail. If the other person did not think your e-mail was urgent when they first saw it, they may have archived your message without answering you. Maybe they have six thousand unread messages in their inbox. Maybe they thought they'd answer you later. As Seth Godin describes in his book The Dip, I consider this behavior to be a natural form of selection: people that don't persist and insist won't get through.
I default to sending a follow-up mail one week after the original, and then one mail each week until I get a response. Sometimes, I send out five or six mails before I get what I want. I keep my messages polite, but short. I don't want to waste people's time. If they are not interested, all they have to do is send me a note with one sentence: "I'm not interested right now." My secret weapon for keeping track of this is Boomerang for Gmail (they have a version for Outlook too).
It doesn't matter if you are looking for a job, selling or buying or setting up a new partnership. It is always OK to follow up on unanswered e-mails.