Spfrules

You can specify a line with local rules.

Local rules means: Rules that are executed before the real SPF rules for a domain would fail (fail, softfail, neutral).

They are also executed for domains that don't publish SPF entries.

I suggest adding include:spf.trusted-forwarder.org.

You can also add mechanisms to trust known mail servers like backup MX servers, though I suggest that you should at least also use tcprules (to modify SPFBEHAVIOR).