Five Indian airports, including major hubs in Delhi, Mumbai, Kolkata, and Thiruvananthapuram, were reportedly placed on high alert following an emailed bomb threat received by IndiGo Airlines on November 12. A News18 report indicated that the threat specifically warned of risks to flights bound for several cities. Livemint stated that it could not independently verify the information contained within this report.
Source: https://www.livemint.com/news/india/indigo-bomb-threat-triggers-high-alert-at-five-major-airports-including-mumbai-delhi-kolkata-car-blast-security-report-11762949109002.html