Email library send mail to spam box

Paul's answer covers it pretty well. I used to manage a fairly large mail server, and the spam software we used had multiple modules to catch spam, each of which had several configuration options and could be set to different priority levels. Setting up proper MX records as Paradinight mentioned may help (especially for SPF), but it's still only one part of the picture. Also make sure your mail server isn't being used by someone else to send spam, because once you get blacklisted by any of the popular blacklists (or by the admin of a large mail server) you'll have a hard time getting email through to anyone.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide