Email / domain reputation is very complex. Their domain may have been flagged due to excessive marketing emails, so no matter what they send they are heavily weighted towards spam.

I worked at a SaaS company that had about 50 sales reps. They were sending emails day and night, eventually getting the main corporate domain flagged. The product's transactional emails (signup, confirmation, etc.) would be flagged as spam, until we moved them to their own domain.

Without more information it is hard to say if that is a mistake.

Sometimes when businesses send sensitive documents they use a third party "secure" service, if the realtor's domain isn't configured correctly (common) to allow that third party to masquerade, it would correctly be flagged as spam.

Plus I get actual spam (and even generated blackmail) with personal information in it (inc. old passwords), farmed from one of a dozen service break-ins over the last 20+ years (thanks Adobe, amongst others).

If the user wanted the email, its a mistake if it went into the spam box, period.

Trust me when i say you can have a 100% flawless email setup in every technical respect (the full works from this article and more) and still end up spam boxed by Gmail approximately 100% of the time.

Conversely, people that Gmail feels are important wont get spamboxed for minor technical reasons either...