PHP Mass emailing

Question

I work for a small marketing company that has asked me to develop a PHP application to send emails for their company without getting blacklisted or emails showing up as spam

Answer 1

If you are sending legit emails you should look into SPF.

Only validates an IP as valid sender of emails from a certain domain. Does not validate content or ensure safe passage through spamfilters. But since spam is illegal in many countries, this validation means they know who to sue/fine.