How does this happen with yahoo mail?

Absolutely not a Yahoo! only thing.

It's a fact that ALL SPAM use faked sender information.

It's hard to make filters with your own address without also suppressing you archival copy and causing some other problems.

Spammers use that in an atempt at bypassing spam filters.

I frequently get the same on my g-mail account as well as that provided by my ISP.

Just flag those as SPAM, then flush yous spam folder.

Electro
July 2012