PHP (mail / PHPMailer / SwiftMailer)

PHP's built-in mail() function doesn't send SMTP directly — it hands off to your server's local mail transport, so pointing it at the gateway isn't a code change at all; see Postfix or Exim, whichever your server runs, to relay at that level instead. If your application sends mail directly via SMTP — most commonly through PHPMailer or Swift Mailer/Symfony Mailer — configure the gateway's settings directly in your application code.

PHPMailer

$mail->isSMTP();
$mail->Host       = 'smtp.adlerwacht.com'; // see /introduction/smtp-relay
$mail->Port       = 587;            // or 465
$mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS; // or ENCRYPTION_SMTPS for 465
$mail->SMTPAuth   = true;
$mail->Username   = 'you@yourcompany.com';
$mail->Password   = 'your-mailbox-password';

Symfony Mailer / Swift Mailer (DSN-style)

MAILER_DSN=smtp://you%40yourcompany.com:your-mailbox-password@smtp.adlerwacht.com:587

Use port 465 with implicit TLS instead of 587/STARTTLS in either example if that's what your setup expects.