Some web servers are allowed to send out plain e-mail using its PHP mail() function. The web server IP may be blacklisted regularly for SPAM since for example some websites hosted on the server does not have web forms protected from SPAM or the client sent out SPAM or phishing intentionaly. Sometimes IP can be removed from blacklist, but very soon after that is listed again and some blacklist managers refuse to remove the IP if it gets relisted frequently.
To allow Your e-mail be delivered when the web hosting server is blacklisted, use SMTP. SMTP should use different e-mail servers that should improve your delivery rate.
1) setup an e-mail account in your hosting account or use third party mail account with SMTP
2) adjust your website code to use provided SMTP server and e-mail account instead of basic PHP mail() function
This way you will have higher chances of mail delivery and work around the hosting server blacklisting.
Send Email via Gmail SMTP Server in PHP: formget.com/send-email-via-gmail-smtp-server-in-php/
Gmail SMTP details: support.google.com/a/answer/176600
Many content management systems like a Wordpress allows easy switching to a SMTP mail sending.
You can also use PHP SMTP mailing solution PHPMailer. Here is a tutorial how to use it.
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