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FWIW I'm also conditioned to ignore emails that are vague. Honestly, at this point I assume any unsolicited communication is a spear phishing attempt until proven otherwise.

Agreed. Extending on /u/formerly_proven point, I think the third variation - where the subject changes - makes the email feel like it's coming from a real human even more.

I think so too. The second test had the same subject line in both emails, though, and the body length was the only variant.