Microsoft added extra security measures as part of the GDPR requirements, which can bounce emails sent using third party platforms that have access to your inbox. Because Outreach connects to your mailbox you’ll need to ensure that you have whitelisted our outbound connecting servers. You’ll also need to whitelist any internal employees that use Outreach so that your Office suite recognizes them as valid senders.
In addition, we recommend setting up Branded URLs. Branded URLs allow you to personalize your public calendar link, unsubscribe link, open, and click tracking links and can improve your spam score by putting these services under a domain that matches your brand.