1) Say you set up a rule that forwards
all of your mail to another address.
What if that address doesn't exist,
and the postmaster at that domain
sends messages back to you telling you
this. Of course, your rule doesn't
make a distinction for these messages
and forwards them as well, creating a
loop.
Well think about it, you'd get back the same "mailbox unreachable" message once, twice, three times. If you just consider it spam and stop applying rules to it after the 3rd identical copy, you're set!
The same applies to your 3rd example too, the autoreply would be identical. Stop running your rules on it after the 3rd identical message.
The 2nd is a bit trickier, and a lot less likely. I'm not sure of a best solution, but you could for example check the differences and see if all the new message is doing is adding a FW: in the title or adding a quote. I don't know..
