 <p>You may be running into <a href="https://bugs.php.net/bug.php?id=69874" rel="nofollow">Bug #69874 Can't set empty additional_headers for mail()</a> if you haven't done anything stupid (i.e. forgot to sanitize the headers).</p> <p>Test for the bug</p> <pre><code>$ php -d display_errors=1 -d display_startup_errors=1 -d error_reporting=30719 -r 'mail("test@email.com","Subject Here", "Message Here",NULL);' Warning: mail(): Multiple or malformed newlines found in additional_header in Command line code on line 1 </code></pre> <p>Alternately if you know your PHP version (hint: <code>php -v</code>) you can check <a href="http://php.net/ChangeLog-5.php" rel="nofollow">the changelog</a> for the bug number (69874) to see whether <a href="https://github.com/php/php-src/commit/dacea3f6fb3f84c79a3412b24547eb2c636754f6" rel="nofollow">the fix</a> has been applied for your version. </p> <p>A short-term fix is to replace calls to mail() like this</p> <pre><code> function fix_mail( $to , $subject , $message , $additional_headers =NULL, $additional_parameters=NULL ) { $to=filter_var($to, FILTER_SANITIZE_STRING, FILTER_FLAG_NO_ENCODE_QUOTES| FILTER_FLAG_STRIP_LOW| FILTER_FLAG_STRIP_HIGH); $subject=filter_var($subject, FILTER_SANITIZE_STRING, FILTER_FLAG_NO_ENCODE_QUOTES| FILTER_FLAG_STRIP_LOW| FILTER_FLAG_STRIP_HIGH); if (!$additional_headers) return mail( $to , $subject , $message ); if (!$additional_parameters) return mail( $to , $subject , $message , $additional_headers ); return mail( $to , $subject , $message , $additional_headers, $additional_parameters ); } </code></pre>