I suspect Gmail is filtering based on content, not based on email service.

Well they're not doing a very good job of it. I disabled this tabbed nonsense right away, because the categorization is shit. They put all sorts of messages under "promotions" that were nothing of the sort, and same for "social". In fact, glancing through the tabs, there really appeared to be almost no coherence whatsoever between what appeared in one tab or another.

As far as I'm concerned, Gmail tabs are totally useless.

Gmail's filtering in this case seems to be 100% spot on for me. It's grabbed all the stuff I consider spam and hidden it away, and hasn't touched anything from people who I want to hear from. A few thousand messages have been properly sorted since I opted in.

Interesting. My experience has been nearly completely opposite of that. Especially in terms of "false positives" where things are being put under the "promotions" tab that aren't promotions at all. :-(

I also really love the multiple inboxes. My inbox is significantly easier to parse.

Once in a while I drag a message elsewhere to train it, but no more than once a month...

The contents of my Promotions tab included many things that were not promotional in nature, including -dev mailing list digests.

Mailing lists are supposed to be filtered into the Updates or Forums tab, depending on whether they are computer-generated or sent by humans.

You can drag the first few emails into the appropriate tab to teach Gmail to do the same for future messages.