@nate as @ibib said, the best option knowing we can’t use a .htaccess file (not apache installed) and the option of customizing /etc/nginx/conf.d/local.conf will force us to re-edit it each time we’ll upgrade MiaB then, the best way I see to achieve this is by using a custom /home/user-data/www/default/index.html file as follows:
<!DOCTYPE HTML>
<html lang="en-US">
<head>
<meta charset="UTF-8">
<meta http-equiv="refresh" content="1;url=https://box.yourdomain.com/mail">
<script type="text/javascript">
window.location.href = "https://box.yourdomain.com/mail"
</script>
<title>Page Redirection</title>
</head>
<body>
If you are not redirected automatically, follow this <a href='https://box.yourdomain.com/mail'>link to /mail</a>
</body>
</html>
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Edit it by replacing box.yourdomain.com by your own server_name.
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Note we are using a meta, and a javascript redirections plus a link inside the index.html static file that should make the desired redirection to work well within any browser.
Hope this helps!