PHP mail() does not work & How to prevent spam?

[7rdoq]

Yeah, just check with mail(), it return false

Then I check sendmail status with cmd 

service sendmail status

it returns

* sendmail.service
   Loaded: not-found (Reason: No such file or directory)
   Active: inactive (dead)

Then I try to send mail with cmd: 

Code:

mail -s "Test Postfix" dfsdfsdfds@gmail.com

but it returns send-mail: Cannot open mail:25