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The disclaimer on the release notification emails is:
"Note: This is an operational email and is sent to all users with a Marketo login."
Therefore, all your users should receive it. It's an operational email, which means it won't matter if they are unsubscribed. Perhaps it goes into their junk mail instead?
How do you handle email hygiene, avoid spam traps? This could be related to your recipients are labeling your email as spam. This tends to be an issue if companies buy a lot of lists and don't run them through email verification or email hygiene or if you data is older than a few years. There are a few products out there that can help with that like Informatica's Strike Iron.