"/" inside an exists: domain-spec?

Looking at the ABNF, I see:

delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="

Does that mean that if a "/" appears inside an exists: domain-spec it's an
error?

Can it be escaped somehow to be legal or is it just forbidden?

I ran across this in testing my SPF test tool and I can't quite see if it's
legal or not:

 -exists:%{i}(_dot_)%{s1(_at_)}(_dot_)100/86400.rate.%{d}

Thanks,

Scott K