I’ve read everything about it there is on the internet. Nothing says about how to set it up for more than one mail server for one domain, or one mail server for more domains, or a combo of those two.
I agree on the ‘service’, just couldn’t think of a better term at the moment.
I also see no difference between MX and SPF records. One is a list of servers that receive mail, and other is a list of servers that send mail. Why the syntax is different one can only guess…