Spam filtering is also something that Gmail does pretty well. If you've got a public email address that's 'out there', a serious spam solution is pretty important.
I've been not very happy with Gmail's spam filtering, though I admit it's hard to do right. But its false positives were the main reason I moved away from it. The last straw was when they spam-filtered an email from my landlord because it contained some Danish text in it (I live in Denmark!). I eventually found it in my Spam folder (which I thereafter had to check regularly) with the explanation that it was filtered because "It's written in a different language than your messages typically use."
I regularly get emails in both Italian and English and it seems to get things right most of the time. And it filters out a ton of spam - I probably get at least 20 an hour, so without a good filter, I'd be "up a creek".
Unless of course you consider multiple daily SEO spams, "spam". Because GMail doesn't filter those at all. Even emails that are worded exactly the same but sent from a different source GMail address.
The answer to spam is to charge a nominal fee for each e-mail, whether in digital currency (Bitcoin, Ripple) or proof-of-work (SHA, Bcrypt, etc.).
Bitmessage integrates the latter (proof-of-work) and is working nicely at this stage, although I believe it may have some structural issues that will make scaling it up to many thousands of nodes difficult.
