I'm an IT technician managing a shared mailbox used by five people through RDS terminal servers. It used to be a regular mailbox with permissions, but we converted it to a shared mailbox after Outlook performance became poor.
Since the change, Reply All sometimes adds the shared mailbox itself as a recipient, causing users to send the message back to the mailbox. Sent messages also occasionally appear in the sender's personal Sent Items instead of the shared mailbox's Sent Items.
The users have Read and Manage, Send As, and Send on Behalf permissions. We have also enabled the options to copy messages sent as or on behalf of the mailbox, and tested removing some of the sending permissions without resolving the issue.
What configuration should we use to prevent the shared mailbox from being added to Reply All and ensure copies of sent messages are saved in the shared mailbox's Sent Items?
2 Answers
Messages sent from a shared mailbox may remain in the user’s personal Sent Items by design. You can’t always prevent that copy, but you can configure Exchange or the Microsoft 365 admin settings to also save copies in the shared mailbox’s Sent Items. Enable the options equivalent to copying items sent as the mailbox and copying items sent on behalf of the mailbox. If the admin portal setting doesn’t take effect, configure the corresponding Exchange mailbox properties for sent-item copying through the Exchange management tools.
The Reply All behavior is commonly caused by automapping. Outlook can become confused about whether the user or the shared mailbox is the active sender. Remove automapping for the shared mailbox, then add or open it manually instead. In classic Outlook, you can add it directly to the profile; in Outlook on the web, use the option to open another mailbox. This can also improve performance because the mailbox is handled separately from the user’s main cache.
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