We (small devshop + some hosting + self-hosted email) hosted a few things for a foundation for years, and about two years ago they migrated the mail stuff to MS. (We continue to host a few sites, domains, DNS.) Now when they need something and send us an email we can't reply, because our IP is "listed".
Okay, I know spam can be bad, and fine-tuning spam filters is a PITA, so let's go through the delisting process, surely with enough perseverance eventually MS will tolerate us into their graces.
Well, it has been more than a year now, and still no luck.
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We have completed reviewing the IP(s) you submitted. The following table contains the results of our investigation.
Not qualified for mitigation x.x.x.x Our investigation has determined that the above IP(s) do not qualify for mitigation.
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¯\_(ツ)_/¯