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...
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).