I have sent an email campaign according to all the terms and conditions, but still my email goes into the spam floder for many of my clients why so?

I have sent an email campaign according to all the terms and conditions, but still my email goes into the spam floder for many of my clients why so?

I make a complete email campaign according to require of all the terms and conditions and sent to my registed customers but when i test it. it comes into the spam floder why so? please explain.

thank you
govind soni