This is definitely a scam. Don't reply to the email at all. Don't forward it or save it. Just mark it as spam/junk in your email client and delete it.
These emails are known as Nigerian scams or "dead customer scams", and there are several variations on this theme. For example:
Someone will email you claiming that they need you to stand in as the next-of-kin in order to access some money. They'll probably ask you to send them various fees, e.g. for processing, bribes, taxes, etc. and in return, they promise you a cut of the funds. Obviously, there is no money. They're trying to scam you out of money or personal information (which they might use to impersonate you and get more money from you).
They might tell you that 90% of the funds transferred will go to you, while 10% will go to the manager. They'll send you a personal check, money order, or even a legitimate-looking cashier check, etc. from a foreign bank and ask you to send them a personal check to the manager (or some "family member" of the manager). They cash your personal check, while the check/order they sent you bounces when it reaches the issuing bank. Not only do they have your money, they also have a check with your personal information, e.g. name, address, bank account numbers, etc., on it.
These are extremely common scams. The safest route is simply to completely ignore the email and delete it immediately.
If you have already replied to the email but haven't sent any personal information, everything I said above still applies. Ignore all further emails from them and delete them immediately. Your email address may receive more spam in the future because by replying, they know that you either have a weak spam filter or are more likely to be fooled by such scams. Regardless, ignore all further emails, mark them as spam/junk in your email client if possible, and delete them immediately.
If you did send any personal information in a reply, monitor all of your bank accounts, credit cards, credit reports, etc. and check for any fraudulent activity. Notify your bank, credit card company, etc. immediately if you see anything remotely suspicious.
Also, never reply to an email like this again. If you have any doubts about an email, ask about it here before doing anything.
DumbCoder made an excellent point too:
Remember when somebody offers you free money, you should ask yourself what have you done to deserve it and that too from a person, about whom you know nothing about.