Improving email delivery (preventing spam flagging)

http://www.emailvendorselection.com/email-deliverability-checklist-improve-email-deliverability-now/

Bulk Emailing using a PHP application http://swiftmailer.org/

HTML email

https://litmus.com/blog/html-email-coding-101-infographic

http://visual.ly/code-therapy-16-tips-troubleshooting-your-html-email