Hi @Vit Kral ,
Based on my test, when I reply to the email, I choose to reply from the shared mailbox, and it can be sent successfully.
In order to confirm whether it is related to the client, log in to the web version and check whether it can be sent successfully. If yes, remove the shared mailbox on the client and add it again to see if the problem continues. Refer to the steps in this article: Open a shared Mail, Calendar or People folder in Outlook for Mac
Also, according to the known issues in the official documentation, the Reply button does not work as expected, please install Office 16.65.22091101 update or later. Follow the steps documented in Update Office for Mac automatically to install the update.
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