In the Mail they noticed the changes about the Monetization. If you have less than 4,000 Hours Watched Time in the past 12 Months you are not eligible for Monetization. OR if you have less than 1,000 Subs also you are not eligible for Monetization. In that case your will Lose your access to all Monetization tools and Features on 20.02.2018.
Means you need minimum 1,000 Subs and 4,000 Hours Watched Time to be eligible for Monetization.
Here is a Detailed Article written by YouTube Team about this Policies = https://youtube-creators.googleblog.com/2018/01/additional-changes-to-youtube-partner.html