Nitin,
as many said above, these days its quite common to issue offers on e-mail to save time and paper. there are positions where people don't stay more than six months or one year specially in BPOs, then they prefer to offer the job on e-mail and ask for acceptance on e-mail only withing stated time frame.
but if someone requires a hard copy of the offer to proceed for resignation in current company, a scanned copy of original offer is mailed to candidates which ideally on company letter head. you can ask for such copy. if it is not possible, tell them that you can collect it personally before joining. go there and collect it, then proceed for resignation.
wish you all the best for your new job!!