It seems that Yahoo!Mail has upgraded its authentication and the currently unmasked version of fetchyahoo v2.10.0 on amd64 does not work with the service (it cannot login). I have unmasked v2.10.5, which so far works ok for me. To reproduce it just attempt to fetch mail from yahoo with the abovementioned version of the software. The error for me was: Logging in securely via SSL as <user> on Tue Dec 3 12:34:44 2006 Failed: Couldn't get challenge to log in. Try again later. This however is not a shortage in service. No matter how later you try the error is always the same, until you upgrade, in which case it works immediately.
Okay 2.10.5 is stabalized and 2.10.6 is also in the tree (~arch).
