Below are all our IP addresses per host, this is useful to, for example, prepare your SPF record if you send your outgoing e-mail through us, or to block the SMTP access to your mail server for incoming e-mail. A good spammer can find out the IP number of your mail server and then deliver spam directly to your server, so outside of our filters! So make sure that your mail server only accepts mail from our servers. If you are, for example, on our cluster03, only give the cluster03a and cluster03b SMTP access.
Pay attention! If you also use our outgoing spam filters, make sure that you create an SPF entry in the DNS registration of your domain (s)! For our outbound01.mx-relay.com cluster, it looks like this: v = spf1 ip4: 18.104.22.168/29 ~ all