By default your Debian server will try to deliver system / Cron emails to your mailbox server directly. While this is convenient for very simple setups, you might run into trouble if your inbox server expects DKIM-signed mails or does not accept emails from every IP address, but only from mail senders with good reputation. For that reason I’m running a single, well configured email gateway server, which is forwarding the mails from all my hosts into the internet. But how to tell all the hosts to send their mails via a central gateway? Installing and configuring Postfix is one way …