Occasionally, you may see a campaign in Inbox Placement with 100% missing at Microsoft Office 365. This typically means the campaign was blocked at that mailbox provider.
But upon further investigation, such a campaign may return a 250ok SMTP code, which indicates that the message was delivered rather than blocked or bounced.
What is happening
In addition to the user-level quarantine you may be familiar with from many filters, Microsoft Office 365 has an administrator-level quarantine folder. Emails that land in this folder can only be released by the IT team (or other administrative team) of the email recipient.
Because these emails are being accepted for delivery, they generate a 250ok response. But they are being accepted for delivery to the admin quarantine, not to the user. So because the campaign—including Everest seeds—is never delivered to the user, it is reported as 100% missing.
Why it's happening
Emails that are selected for administrative quarantine usually are interpreted as phishing or malware, but can also be spam or bulk emails. If your campaign is falling into this category, review your list hygiene, content, sending volume, and cadence to make sure you're adhering to best practices.