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It is not uncommon for a mail IP address to end up on a public blacklist, especially on a shared server. It could be due to overall volume of mail coming from that server, or messages seem to have characteristics of spam in them.
Another common cause is email forwarders. If you have firstname.lastname@example.org forwarding to email@example.com, firstname.lastname@example.org is spammed, our mail server could forward that onto Yahoo.
Their servers might see our mail IP address as relaying the spam message to their server, even though it wasn't the originating server of the spam.