There are several ways to improve the chances that your email messages land in Outlook Inboxes instead of Spam or Junk folders:

1. Ensure the use of valid DNS records. Pay particular attention to the SPF record.
2. Enforce TLS encryption for sending and receiving messages.
3. Authenticate with DKIM, DMARC and MARC.
4. Make sure your custom domain isn’t on any public blacklist.
5. Ensure the quality of recipients and messages.