It seems that several of the large web based email services are being blocked by realtime blackhole lists. This article only mentions gmail but I've noticed it with Yahoo! mail as well.

Over the past month, major anti-spam vendors have had to apply scrutiny to Gmail in a way they haven't had to before, and the result is reduced delivery performance and sometimes outright blocking of Gmail.

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Gmail being throttled, blocked by some anti-spam vendors

I've had to make adjustments to mail servers to allow legitimate mail to pass but a larger volume of spam is passed as well.