SPF issue: what causes softfail?

Question

Whenever I use PHP to send emails to members of my site, the email ends up in the spam folder for most email providers like Gmail and Hotmail.

When I check the original

Answer 1

That means you are sending mail using email address which is not yours (e.g. your smtp server has domain name CompanyABC.COM while your're sending email whith sender: someone@CompanyDDD)