If your server is having ipís from xxx.xxx.x.11 to xxx.xxx.x.20 , you can use above line.
+1 is is the starting ip ie xxx.xxx.x.1í1′ and %9 is the increment value ie 11(xxx.xxx.x.11) to how much. Here it is 20 (xxx.xxx.x.20)
it was said by p-root that it is not working anymore on latest cpanel version. haven't tried this though, how can this be done if your IP pool is broken like this:
xxx.xxx.xxx.146-148 and xxx.xxx.xxx.160-165
You might want to look at cPanel's options for sending emails from their site IP. This works reasonably well and means if someone does something that gets blocked it only affects them.
Then, put your high risk domains on another IP.
I'd think your IP rotation scheme, while really really clever and ingenious*, would lead to possible random IPs being blocked and then email will fail randomly, creating a support nightmare. I suspect this doesn't happen much in real life due to the lack of outgoing spam, but it's possible, and messy.
You could also use an auto-alert script or website - there are several that will send alerts, and a simple one at www.whmscripts.net (our site) which will email you if any of your server IPs are found on a set of nominated blocking lists.
With latest version of cpanel you can simply change the IP specified in the /etc/mailips file.
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