If the emails you send from Outlook end up in the recipient's spam filter there are various thing you need to check.
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If you follow this link you can read Microsoft's own guide to best practices:

Bulk Emailing Best Practices for Senders

You can always spam test you email on this page: https://www.mail-tester.com/ and get general advice on how to avoid your emails ending up in spam here:

Emails end up in spam (email template)
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