The absence of an SPF record ought NEVER to be used as a justification for routing something to SPAM. Do you have any clear evidence that it is SOLELY the absence of a SPF record which is routing items to SPAM?
When a SPF record IS PRESENT then if the relaying SMTP server does not match the SPF record, then that is fair grounds to suspect and treat as SPAM. In the absence of a SPF record, the validation should fail-safe to "neutral".
Note that where SPF is implemented, and the email is 'passed on' via a forwarding service (for example fred.bloggs@nowhere.com auto forwards to Fredrick@somehwere.com) even if the sending address does have SPF set, if the forwarding service does not have SRS (see https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme) enabled, then the end service might well reject the email as SPAM.
SRS was devised in order to forward email without breaking the Sender Policy Framework (SPF), in 2003
In a number of cases, including change of email address and mailing lists, an MTA accepts an email message that is not destined to a local mailbox but needs to be forwarded. In such cases, the question arises of who deserves to receive any related bounce message. In general, that is either the author, or a person or other entity who administers the forwarding itself.[2] Sending bounces to the author is administratively simpler and used to be accomplished by just keeping the original envelope sender. However, if the author address is subject to a strict SPF policy (-all) and the target MTA happens to enforce it, the forwarding transaction can be rejected.
In other words, SPF being enable could cause more issues than it solves.
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