Happy 6Eteen, IndiGo - How Akasa Air congratulated IndiGo on its 16th birthday
6E is the airline code of IndiGo. Every airline in the world has a designator code. For example, Go First's code is G8, Air India's is AI and Akasa Air's is QP

In response, IndiGo thanked the Rakesh Jhunjhunwala-backed carrier. Source: Reuters