Hi Simon,
The fastest first fix is to allow-list the sender so Outlook always accepts those messages.
Sign in at Outlook, open Settings > Mail > Junk email. In Safe senders and domains add the newsletter address or its domain, for example news[at]example.com or example.com.
In the same Junk email page make sure the sender or domain is not listed under Blocked senders and domains, remove it if you see it, then Save. Send yourself a test from another address or ask the newsletter to resend, and check both Junk and the Other tab if you use Focused Inbox.
If messages still do not show up after this change, tell me which Outlook you use, web app or desktop, and I will guide you through the next check.