Koinex is a legitimate website and I have been using this for a few months myself. However, it still faces the common problem every other provider in India does - delay in providing support - this is because the volume of users in India have been burgeoning and there are not enough cryptocurrency exchanges to support demand. https://ethexindia.com/ is the oldest Ethereum exchange in India and is even slower.

They don't give out an email contact but provide a contact page - https://koinex.in/support . I can suppose with the volumes they are managing now, they wouldn't want a flurry of emails.
Koinex is also among the most widely used exchanges by the members of the largest crypto FB group in India - https://www.facebook.com/groups/BharataFinance/ and the relative feedback on it has been good by those who have used it regularly.

With respect to reliability, all the popular exchanges (Zebpay, Koinex, EthxIndia, Pocketbits) are all in the same boat - they are simply not able to handle the volumes of KYCs these days and hence take a long time to process. While I had my KYC processed in Koinex in just a day or two, people now wait a week ore more. However, once you get this complete, things are smoother.

Zebpay is the fastest from what I've seen so far, but they only provide an option to buy Bitcoin and for Ether, you need to transfer your bitcoing to an exchange (or a website like changelly) to convert which would involve needless transfer charge.

PS: I never recommend anybody hold their investment in Koinex, or any exchanges, as mentioned in the article. Use them only for buying and selling, and not as a wallet.