Sending, receiving, and organizing email in Outlook on the web for business
If the missing messages are also not visible in Outlook on the web (OWA), this generally indicates that the email might never reach your mailbox.
To help determine where the issue is occurring, could you please provide whether those senders received any bounce-back or error message after sending.
Additionally, please ask your Microsoft 365 administrator to:
- Review Message Trace in the Exchange Admin Center for the affected sender(s) and recipient address to verify whether the messages reached the tenant.
- Verify any NDR (Non-Delivery Report) received by the sender. The exact error code and diagnostic information in the NDR can help identify whether the message was rejected, blocked, misrouted, or failed for another reason.
- Check whether the messages were quarantined, filtered by mail flow rules, or impacted by Exchange Online Protection policies.
If Message Trace shows no record of the message and it is not present in Outlook on the web, that would suggest the email likely never arrived at the mailbox service, and the investigation should focus on the sending side or the mail flow path between the sender and recipient.