sending email with PHP (preventing from being placed in spam folder)

Answer 1

In general, email is classified as spam or not spam on the receiving end, not the sending end - otherwise, spammers would simply say that all of their messages aren't spam, completely defeating the purpose. Thus, you can't just force a message to go to a sender's inbox.

However, what you may need to do is see if the machine that you're using to send mail is currently listed on any spam blocklists, and if so, take the necessary steps to remove it from those blocklists. The most common is probably Spamhaus.

Answer 2

How to increase the chance that the receiver doesn't mark an email as spam has been discussed quite some times here on SO. E.g.

http://stackoverflow.com/questions/371/how-do-you-make-sure-email-you-send-programmatically-is-not-automatically-marked

But there may be some tips specific to pear's Mail package on how to implement these steps (so I wouldn't consider this a duplicate ...yet).