Cash deposited - colleagues salary

Hi,

I recently received a mail from IT Dept that I had deposited around 16Lakh cash in my SB account during the FY 2015-2016, and the reason you have not filed returns.

I am a staff in a college and they paid all the staff through cash. Our college is in the outskirts of the city. We were not paid monthly, due to some scholarship issues from the Govt. They used to pay us at once for 3 months or 5 months. 

I am having an account in a bank nearby our college. So some of my colleagues gave money to me which I deposited into my account and then transferred into theirs. 

Now after going through the transactions for FY2015-2016, the amount I transferred to other accounts is in total of 13 Lakhs. The remaining 3lakh include my salary and some amount from my father,sister and wife. I have all their PAN Cards and account numbers. 

My salary is 18000per month. 18000*12=216000. Around 1 Lakhs rupees my sister transferred to wife and she inturn transferred to me. Do I have to show account for all the online transfers I received also?

As I am well below the tax slab, how should I proceed with the reply to IT Dept?

Please advice.