Secondary Mail Servers - any docs or tutorials?

Hi,

I have been looking for docs but cannot find anything..

I am trying to determine exactly what the Secondary Mail Servers feature is and how to use it. I have an 'email server' that i use for marketing and would like to be able to share the load with another server but am not sure if this is what i would need. Also how will that affect RDNS / SPF / DKIM and any other potential mail issues..

Any guidance and details would be appreciated! Thank you.