Some users not receiving emails sent using mail()

[Zaggs]

Hi Guys,

I am having some issues with users not receiving emails sent from PHP function mail(). Some users are receiving the emails and some are not receiving them at all.

Here are the headers that I am using when sending the mail:

From: noreply@mysite.com 
Reply-To: noreply@mysite.com 
Return-Path: noreply@mysite.com 
Content-type: text/html X-Mailer: PHP/5.2.6 

Should I add anything else?

[AnthonySterling]

Hi Zaggs,

From experience, I've found trying comply with every (major) mail servers spam rules almost impossible.

Have you though about using a mailing library such as SwiftMailer etc? I often find these are far better at getting through spam filters than supplying my own headers.

[crmalibu]

http://swiftmailer.org/wikidocs/v3/tips/spam

[thewebhostingdir]

Some mailserver rejects the emails if they are unable to find the smtp authentication attribute in the headers. This might be the case in your issue.

[lampcms.com]

What is this:
Content-type: text/html X-Mailer: PHP/5.2.6

Do you have 2 headers on 1 line? This is bad

I recommend to remove X-Mailer header completely.

Also try to send in plaintext format and see if that helps