You must have a plugin that affects how WordPress delivers email.
Are you talking about any plugin for email? If yes, I am not using any plugin for send mail but I am using SMPT mail with override wordpress “phpmailer_init” function
See below function
add_action( 'phpmailer_init', 'wpse8170_phpmailer_init' );
function wpse8170_phpmailer_init( PHPMailer $phpmailer ) {
$phpmailer->Host = 'smtp.gmail.com';
$phpmailer->Port = xxx; // could be different
$phpmailer->Username = 'xxx@gmail.com'; // if required
$phpmailer->Password = 'xxx'; // if required
$phpmailer->SMTPAuth = true; // if required
$phpmailer->SMTPSecure = 'tls'; // enable if required, 'tls' is another possible value
$phpmailer->IsSMTP();
}
