why my emails are going in spam folder

I don't know where to ask this question ? I know it is in wrong section.

I am building a list from facebook followers and sending email. They like my email content. first day I got 30% reply. But nest day my all mails goes in spam folder.

I am sending mail using php mail() function. I fill email content, it take in variable and get email list from database send them one by one. all mails are send in less then one minute.

there is anything wrong in my process.
Thanks

//Edited
one thing more I am sending HTML email.

Blacklisted IP?
Content w/ trigger words?

Could be a lot of reasons.

AnotherCoder said:

Blacklisted IP?
Content w/ trigger words?

Could be a lot of reasons.

Click to expand...

I send email with same subject. It may be issue. some part of email was common at top and bottom.

Possibly the amount of emails you send per hour?

j3ss3jam3s said:

Possibly the amount of emails you send per hour?

Click to expand...

I sent 100+ email first day. after 24 hours I sent 200+ emails. all emails goes in less then one minute.