Our newsletter lands in spam for every Hotmail and Outlook address

Two smaller things on top of the authentication side: - ask a handful of loyal subscribers on Outlook to move an issue out of junk and add you to safe senders. it doesn't fix the cause but it gives their filter positive signals - make sure the one-click unsubscribe header is actually working. people who can't unsubscribe easily hit junk instead, and that's the worst signal you can get if you warmed up a new domain or IP recently and went straight to the full list, that alone can do it too. ramp up over a few weeks.

Microsoft is stricter than Gmail about this. First thing, make sure SPF, DKIM and DMARC all pass and line up with your sending domain, not just your email provider's default domain. Send yourself a copy and open the message headers to check. Then sign up for Microsoft's SNDS and JMRP programs so you can see your IP reputation and who's marking you as junk. If you're on a shared IP from your ESP, someone else's sending can drag you down, so it's worth raising with them. Clean out people who haven't opened in months too, they hurt more than they help.