First thing first. You will need a visa to do this itinery. The del -Maa leg is a domestic flight for which you will have to clear immigration and customs at delhi airport. This itinery is not going to work without a visa.

Secondly, you can try emailing them at [email protected]. It's the email address for their online booking system. I can't assure you that this problem will be taken care of but it's the best way to address it.

Thirdly I will suggest you alternate itineries that will actually work but again I am not able to book this on air india website as a single ticket. Take the kbl del flight and thereafter take the del bom flight with flight number 317/315 which departs at 2250 from delhi to Mumbai. This is an international flight and you can use international transit at delhi airport. (Do not book any other flight from del-bom as all others except 348 and 102 are domestic flights.) This flight gets you into Mumbai airport at 0055. Thereafter, you can take the srilankan airways flight from bom to cmb which departs around 3 in the morning. An indian visa will not be required for this itinery. Air India has code share with srilankan.

Option 2 is to take the spice jet flight from kbl to del and then take the del cmb srilankan flight. This will be the shortest itinery however, I don't think your bags will be interlined between the 2 carriers and I am not sure how you can do the transit without a visa at delhi for this itinery.

Hope this info helps.