Campaign emails may disappear from recipient inboxes and then reappear days later as new messages with current delivery dates. This happens when Microsoft Defender quarantines and later releases emails—a process that occurs entirely outside of ContactMonkey.
What's Happening
Microsoft Defender may quarantine emails after initial delivery if it flags the content or linked sites as potentially malicious. When Microsoft or your Office 365 administrator later releases these quarantined emails, they reappear in inboxes as new messages with updated delivery dates.
This is not caused by ContactMonkey resending campaigns. ContactMonkey cannot delete or redeliver emails after they've been sent.
How to Verify This is Happening
Check the original recipient's mailbox to see if the first successfully delivered email is still present. If it's missing, Microsoft Defender likely quarantined it.
Microsoft 365 administrators can confirm by checking the Microsoft Defender Quarantine Page and searching for the affected emails.
How to Prevent This Issue
Adjust your Microsoft Defender policies to reduce false positives for your campaigns. Review our article here for specific policy recommendations.
If you want Microsoft to change how quarantined emails are handled when released, submit feedback directly to Microsoft through your Office 365 admin portal.