We scraped Yahoo Mail for about 6 months quite heavily. We had to keep it at a >5 minute timer otherwise we got captchas during the auth process, or we got locked out for 24 hours with error 999.

We now have a formal agreement with Yahoo, but during the process Yahoo indicated they had an informal open policy on scraping. Note that they have an initiative to open up all services within the next year or so (google Yahoo Open Strategy to read more about timelines).