Those INSERT guys who’ve been talking about a GMail security hole allowing spammers to relay spam, have released more previous-redacted details here. (thanks to the MailChannels blog for pointing that out.)
In essence, the attack works by allowing a spammer to set the “forward to” address in GMail to point at a target address, send a spam to the GMail account, then change the “forward to” address to the next target and repeat.
it’d be trivial for Google to impose stringent rate limits on “forward to” address changes, and I’d be surprised if they haven’t already.
ditto rate-limiting on the rate of forwarding messages for each GMail account.
as they say in the paper — if Google required up-front confirmation of the target address before forwarding any mail, that would also cut this out neatly.
It’s worth noting that GMail’s outbound servers may be whitelisted by some recipient sites, others are treating them negatively — word on the anti-spam “street” is that GMail is becoming a festering pit of 419 scammers these days.