[cj]

I don’t disagree with you, but before assuming it’s the fault of gmail classifier I would look at Google Workspace admin configuration. There are a lot of settings that admins can tweak and toggle that can mess with email deliverability. You can even create specific rules that only apply when users within the same workspace are emailing each other.

Google Workspace can even be configured to use an external smtp service behind the scenes. Can also be configured to proxy emails through 3rd parties (in which case the email might be leaving the Google ecosystem and then reentering it from a non-Google IP). There’s a lot of silly (seemingly unnecessary) features on the admin side that could trip up a spam filter.

[JimDabell]

In this case it was a super-basic setup. Nothing weird going on, just an internal email within the same Google account, sent from the Gmail web interface and going straight to the spam folder for the recipient, no filtering rules or anything like that.