Many people don't fish up the unsubscribe link from the bottom of the mail when they don't want to receive your blog posts anymore. They just click that nice prominent "spam" button in gmail/outlook/their mail client.
It's a super convenient solution for them, but if you have a low-volume mail server and a few people do this to your mailing list it's very easy for your entire domain or the server IP to end up blackholed at Google or Microsoft services. That's ok if it happens to your blog, but if all your personal mail ends up undeliverable it can be a pretty big problem.