I just found that I couldn't use a xoxy.net address
-- i.e. from.yahoo.20.psst@xoxy.net --
with Yahoo because they block that domain.
I tried some other spamgourmet domains - still no luck.
When I tried to register the address with an extra 'o'
-- i.e. from.yahooo.20.psst@xoxy.net --
the domain was no longer a problem.
I tried a few more variations and convinced myself that their algorithm searches for ".yahoo." in the e-mail address and then refuses to register it, stating that the domain is unacceptable.
Maybe it isn't sneaky and it is just a bad error message. After all they could have got me to verify the address and then blacklisted the entire domain for real. On balance I prefer their algorithm to this really sneaky one.
Anyway - I thought it was sneaky enough to warrant joining the BBS and pointing it out.