Originally Posted by 678flyer

This doesn't work that way in Delhi for Air India. It depends whether the next flight is an "international" flight or not. In the case of AI, any flight number starting with 0, 1, 2, 3 or 9 (assuming a 3 digit flight number) is treated as an international flight even if the leg is purely domestic. If the connecting flight falls in this category, then immigration & customs is at the final destination. If not, it is on arrival in Delhi. This is explained in every issue of AI's in flight magazine. Here is the link to the March issue.

In the case of the OP, it is a "domestic" flight with a flight number between 400 and 899 (and a 15-hr wait) so they will have to get immigration & customs clearance done in Delhi.