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I found my yahoo mail account often received junk mails requesting me
to do some fund transfer from african countries. Some of them could be
identified as junk mail by yahoo but some are not (even they come from
the same sender or domain). I would like to ask:

1. How can I change the settings such that all of them could be
identified as junk mail (yahoo couldn't give me satisfactory answer,
more accurately speaking, they rarely answer questions sent to them!)

2. How those mail sending applications use tricks to bypass junk mail
filtering mechanism employed by large ISP or web site ? Any web site
has discussion on this issue ?