Maybe it's a nice feature for the next version of ISPconfig?
Something like some preset SPF records option, where the admin can make a selection of some preset SPF's?
option1: Allow domain's MXes to send mail for the domain, prohibit all others.
v=spf1 mx -all
option2: The domain sends no mail at all.
option3: The domain owner thinks that SPF is useless and/or doesn't care.