One way to look at this is: Which choice benefits the user most?
Since the Yahoo mail is being forwarded to Gmail, I would think it
would be more valuable to have Gmail doing the spam filtering.
Why? If _either_ of them blocked it, the user wouldn't see it.
Training Yahoo helps the user if he later switches to having someone
else pick up his mail from Yahoo. Training google doesn't.
Training google to see mail forwarded from Yahoo as spam can have
adverse effects for him.
Firstly, I think the scenario didn't talk about forwarding, but rather about
fetching email using POP3 (or possibly IMAP). Secondly, if Gmail did receive
forwarded mail and then considered a click on the TiS button (which has always
been there in Gmail) to automatically mean "this IP address is a spammer", then
it's doing something wrong. (And given then Gmail itself can be set easily to
forward email, they're unlikely to act this way.)
So it's better to train Yahoo. (Now, if you believe that google
trains much better than Yahoo, that's an important difference.)
But the only thing your MUA can be sure about is where it received the email
from (Gmail's IMAP). It may have a good reason to think that Gmail fetched the
email from Yahoo, but it has to trust Gmail for that. Which might be okay if
Gmail is really Gmail and Yahoo is really Yahoo, but not if Gmail is a random
IMAP server $x and Yahoo is a random POP3 server $y and you as an end user
haven't explicitly told the MUA how it receives email.
So I think the only right thing to do is to send the report to Gmail.
Gmail (or $x) then knows that (and how) it received the mail from Yahoo (or $y)
and hopefully knows it is best to forward the report.
Martijn.
Virus Bulletin Ltd, The Pentagon, Abingdon, OX14 3YP, England.
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