Depends on the situation really.
If the domain is not listed for sale anywhere then your email is unsolicited and as such, it being ignored shouldn't be a surprise to you.
If it is listed for sale, and you are using the correct contact details, then they are either not receiving the email or are receiving too many emails, or maybe they are just waiting to see what enquiries they get and will filter and respond to all after a period of time.
I agree with PORNMD about not automatically dismissing gmail, hotmail etc. If I was enquiring about a domain, it is unlikely I would use an identifying email in the INITIAL approach and to expect somebody to present credentials and price they are willing to pay in that first email is just not realistic.
No matter how great the domain, nobody wants to pay more than they have to.
If domain not listed for sale, I would start with something like this:
"I understand you are the owner of x domain. I do not know whether you are interested in selling this domain, but if you are, I would appreciate if you could give me an idea of the kind of price you would be looking for.
If you are not considering selling at this time, my apologies for troubling you, however if this changes, please feel free to contact me."
If they didn't reply to that, well then they are just downright rude
