Key points:

1. Also deliver email to hotmail using a mail server that has the required reputation and sends large numbers continuously.
2. Test email sending with tools, such as on this site, so that a fake sender does not pass through a spam filter.
3. Multi-tenancy (rental of software on a web server) requires the ability to arrange multiple mail services (mainly SMTP).
4. Forwarding of mail does work with a mail service with SRS technique (Sender Rewriting Scheme).
5. For outbound email internet.nl can only test for validity. A mail server can look suspicious in case of suddenly much more email.