score:6
I do domestic to international connections through T3 at Delhi Airport often.
While the answer by user DumbCoder is correct, it needs a bit more information to be complete, as mentioned by user Mophotla in the comments.
There are two scenarios to consider here: whether the first leg is a part of a longer international flight, or a domestic only flight.
In this scenario, you are flying from BOM to DEL in a flight that either originates or terminates at an international destination. An example of this kind of flight is AI101, that goes BOM-DEL-JFK. In this case, passengers travelling only on the BOM-DEL section of the flight (or those connecting forward for a domestic destination) will be given a boarding pass that has a big letter D on it. In your case, since you will be transferring to an international destination, your boarding pass will have a big letter I on it.
Since it's the same aircraft and boarding will take place from the same gate, passengers only travelling domestically will have to go through a quick immigration control, in which only the boarding pass is looked at. If this scenario takes place, in your case (with the letter I), you will have to go through exit immigration in Mumbai itself and only go through a transfer security check in Delhi.
In this scenario, you are flying from BOM to DEL in a flight that is solely a domestic flight. An example of this kind of flight is AI806 (as mentioned by user Mophotla). In this case, you will simply go through the domestic security in BOM, arrive in Delhi, and be redirected to the international gates. Here, you will first go through immigration and then transfer security. You will have to follow the signs for international transfers in Delhi in this case. There will also be Air India staff when you disembark to take you to your connection.
Upvote:7
I flew from Kolkata to London. My immigration was done in Kolkata and in Delhi I only had a security check and a cursory glance at my passport/visa near the departure gate by the person manning the entrance to the international departure gates.
Regarding timelines, you should be fine unless your arrival flight is awesomely late. If you are checked in they will try their best to get you on your flight else the next flight like all airlines. Assumption is it is a single ticket.