The Issue
A user reported that a Microsoft 365 shared mailbox was constantly receiving out-of-office (OOO) replies from internal staff.
This was identified during a routine IT support investigation for a small business client in Oxford.
Initial checks showed:
- No mailbox forwarding configured in Exchange Online
- No obvious mail flow rules
The Investigation
Using Exchange Online message trace, we identified the following email flow:
- An external email was received by the shared mailbox
- A second message was then sent from the shared mailbox to a user
- The user had an out-of-office enabled
- The OOO reply was sent back to the shared mailbox
The Findings
The cause was an inbox rule in the shared mailbox:
- Emails matching certain keywords were automatically forwarded to a user
Because the email was forwarded from the shared mailbox, it appeared as the sender, so all automatic replies were returned to the shared mailbox.
The Resolution
The rule was identified and disabled, resolving the issue and stopping the out-of-office replies.
Key Takeaway
If a shared mailbox in Microsoft 365 or Exchange Online is receiving unexpected out-of-office replies, check for:
- Inbox rules
- Power Automate flows
- Internal email forwarding or automation
This type of issue is quite common in small business IT environments and can usually be resolved quickly with the right investigation.
IT Support in Bicester, Oxford & Oxfordshire
If you’re experiencing similar Microsoft 365 email issues, AGGIA IT Services provides reliable IT support in Bicester, Oxford and Oxfordshire, helping businesses troubleshoot and optimise their systems.
