Proper etiquette for emailing

I'm wondering if any other vendors are running into this problem. I'll get an email from a bride, asking questions and whatnot, so I email her back but don't get a response. Is it ok to email a second time, saying that I just wanted to make sure they received the email? How long should you wait between the first and second email? I don't want to seem pushy or desperate.

Thanks

Christi