Authenticate Email PHP

I have a web app which needs to send emails to clients 'From' staff email addresses. What's the best way to prevent my messages from being flagged as spam?

For instance, if I own charles@gmail.com, I'd like to be able to send mail "From" that address with PHP in my App, without getting the "This message may not have been sent by...." message.

Right now I'm just using the mail() function within PHP, with Headers for the From, Return Path, and X-Mailer variables.

I'm generally pretty confused by everything I've read so far about SPF and DKIM, so I appreciate any advice. Thanks.

Answer

This is a very lengthy subject with lots of things to consider.

The most important rule is to not use HTML and to send only correct mails that people want, and that the recipients do not flag as spam theirselves.