U may have to disclose how u got this correspondence, was it thru a mail or letter head paper? If though a mail, the authenticity is in serious doubt. however, if you got it thru the official letter head of the recruitment firm, then obviously they must have an address which is verifiable.
But in summary anyway, I still think any reasonable and straight-forward firm will first have dealings with u physically before asking u to pay any sum, so BE CAREFUL!