[felix]

[Jerry Atrik]

we have yahoo blocked because we were getting 1000 spam emails a day
(yes one thousand)
one of our customers also block all of yahoo email.

instead of more size they should fix that

[felix]

Where the e-mails actually coming from Yahoo domains or was it that the spammers were spoofing their addresses. A few months ago I was receiving many "undeliverable" auto-responders as someone was spoofing my Yahoo address to send spam.
You can't stop that happening but you could get your mail server to determine if the domain on the e-mail address matches the source domain. Where it doesn't match, can the message.

ie - message from xyz@yahoo.com, originated from xx.spammachine.dodgydomain.com therefore kill die death, however if message from xyz@yahoo.com originates from xx.xx.yahoo.com then it's okay.

Alternately you could find out who is sending the spam and pay them a visit to hack off their hands with a reem of paper.