Hi @Cloud_Geek_82 ,
Welcome to our forum!
According to your description, I noticed that you have set the licensed shared mailbox as additional mailbox in Outlook profile and made it as default account. As I know, to make it to be default for outgoing emails, you need to select the "Always use the default account when composing new messages" option via File > Options > Mail>Under Send Messages.
In this way, no matter whether you select the default account before creating a new email, the "From" field in the new email will display the account by default.
Hope to help you!
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