[PHP] Automated Emails

My emails sent from my registration script to verify accounts are ending up in spam folders, and in some cases, not even being put in the client's inbox at all.

I'm thinking that it's a problem with the headers. I remember trying out a sample PHP email script and running it on my server, and it ended up in my GMail inbox (not spam folder). I deleted the script, can't find it, and now I have my own script, which I'm pretty sure has different headers.

So what headers should I use to make sure my emails don't end up in the spam box?

I've had some success with the xmailer header:

Unfortunately, this could also have to do with your mail server's dns info or list status (black/white).

Another thing to do is audit your content for common filtered items, check here for the list of spamassassin targeted items.