How can one ensure that emails sent from PHP do not get marked as spam by email providers like GMX?

To ensure that emails sent from PHP do not get marked as spam by email providers like GMX, you should set up proper email authentication using SPF, DKIM, and DMARC records for your domain. Additionally, make sure that the email content is not spammy, use a reputable email service provider, and avoid sending emails in bulk. 

// Example PHP code snippet to send an email with proper authentication
$to = "recipient@example.com";
$subject = "Test Email";
$message = "This is a test email.";
$headers = "From: yourname@example.com\r\n";
$headers .= "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/html\r\n";

// Set the sender's domain for SPF authentication
ini_set("sendmail_from", "yourname@example.com");

// Send the email
mail($to, $subject, $message, $headers);

Keywords

PHP email spam deliverability authentication

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