1 One Click Gmail MX (done)
2 One click Outlook MX (can be more complicated to integrate)
Other providers might be suitable to be added to the list if required.
a If SmartHost is used, Relay should be included in SPF of all domains.
b If Gmail or Outlook Remote Mail is used, Respective "Include:..." should be in the SPF of the domains
Settings can be done manually, I agree!, but some help from the UI is welcome.