I was trying to track down why my email just appeared to melt away over the last few days. I checked the usual reason, my ISP, but for once they were blameless and actually helpful.
I eventually realised that it must be Yahoo! SpamGuard that was declaring each and every email I was getting as spam. Once real emails are classified as spam then they can very easily get lost in the bulk folder, as many spams are post dated, leading properly dated emails to appear on about page 19 - and there is no way to sort by sequence. So to get anything back I have to do some imaginative searching, for emails I think I might have recently recieved.
Even now that I have retrained SpamGuard not to classify emails from myself to myself as spam, they still end up going back in the bulk folder eventually. The reason for that is that spammers send thousands of emails claiming to be from me, and these get bounced as spam by ligitimate system admins, then SpamGuard associates emails from me with being spam.