So I get an email which does not want me to buy anything but just wants me to answer
What is this? What is the purpose? Just to verify if my email address is valid?
Interestingly, I looked at the header and it’s originating IP 25.49.123.51 belongs to the ROYAL SIGNALS AND RADAR ESTABLISHMENT of the British Ministry of Defence. What is the Royal Establishment up to these days?
If you reply, they know that this is a live email that they can use for future spam. Possibly it is a set-up for a Nigerian type scam.
Haj
Somewhat relevent to the op, from this BBC story:
The “product” of spam isn’t penis enlargment pills or whatnot, it’s your address. The main, if not only, purpose of the message is to get you to respond to it.
Apparently not closing their relays…
You know, I’d feel really stupid if it turned out a 26 year old British royal really was after me and I missed the opportunity. 
The link posted by jovan says spam is 40% of traffic (more like 96% of filtired email in my box which must mean something like 99.6% before filtering) and that spam is 25 years old. I don’t know what they are waiting to do something about it.
I am still somewhat suspicious of that email. I don’t think mailing to random addresses and waiting for responses is a good way to harvest. I wonder if there’s something more to it.
They could have bounced the email off the British Ministry of Defence. According to this site
Notice that the email spells sights as sites. Little too Freudian there.
To add something to the discussion, I’ve gotten spams that advertise what’s known as `leads lists.’ The spam promises a huge list of emails that are known good and at least somewhat responsive to spam. So I suppose that email was sent by a leads list compiler, testing the waters for live addresses among all those he’s harvested from Usenet and websites and such.
Remember that spam is a game of small percentages but big numbers. One percent of a hundred thousand is still a thousand, and if you can get thirty percent of those yutzes to send you fifty bucks, you’re pulling in fifteen grand for practically no work. Such is the New Economy.
When I had an email address that had my last name in it I would get lots of “offers” like that, and also “family chain letters”.
All would claim they were looking for relatives to fill out family trees, but soon you’d start getting ads to buy things with your family crest on them!