Is 48 minutes enough time to go through custom and check in to my next flight ? Enough for the airline to transfer my luggage ? I am using Delta from NY and next flight is also Delta to seoul.

I think you can make it in time since you are already there...Had you not been, then I would have recommended twice that time...Hustle up and you are on you way...Safe and happy travels.

Tight

When you're checking in in New York, how's your immigration stuff should be done there - passport visa/check and all that - presuming you are American or a legal resident. If you are having to do your exit visa because you're a visitor... I have no clue. Physically you should be able to make it from whatever concourse you land to whichever concourse you depart from even the f terminal - since it's probably a large jet from Atlanta to Seoul. Your bags might have a challenging time making it... So be sure to pack a little bit in your carry-on in case you get stuck without your bags for a day. It will be tight though. on the bright side there are tons of flights from New York to Atlanta and I will consider getting on one that arrives an hour earlier just to be safe.... And deal with delays so you don't miss your keystone flight from Atlanta to Seoul.