My new server runs qmail, which I've never used before. There is no inbound mail on the server (all the clients have mail hosted elsewhere, but some systems on the server send mail. I'm about to move a client there who has several parked domains, and looking at the smtp log, the server is already blocking many attempts to relay spam using one of the parked domain names (probably because a related domain is already hosted there).

How do I ensure those mails stay blocked, while allowing legitimate addresses to send out?

Server OS is CentOS and hosting software is Kloxo.

closed as off-topic by HopelessN00b Apr 9 '15 at 19:34

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It depends how QMAIL was installed on your particular server. The thing is, naming conventions and installations are going to vary. I have my own build I compile from source and am not going to make assumptions about your configuration.

If you look at the process list, you will see the entry for qmail-smtpd probably using tcpserver. You can specify the RELAYCLIENT variable for certain hosts to allow relaying. It is a binary database, so after updating the flatfile you will need to recreate it and then restart the tcpserver instance running qmail-smtpd.

The documentation on the QMAIL Web site called "Setting up servers" describes this in detail. If you encounter a particular issue and describe it in detail, I'll likely be able to help.