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I have never received any important email with emoji in the subject. Actually, I find them so unprofessional that I send them straight to spam.

Not just unprofessional, actual spam is the only place I've ever seen this. It's the clearest signal to me to delete without reading.

Good point. If someone sends you valuable information, he don't have to try to steal your attention with such cheap tricks.

Counterpoint: eBay order updates use emoji in the subject. I don’t necessarily like it, but it could be considered important email.

I don't use eBay, so I don't know how these mails looks, but I bet they treat it as a marketing opportunity.