Thanks for clarifying. In that case, if the other mailbox is added to Outlook as an additional mailbox (eg within the settings of your main mailbox) then it's not possible to make that the default with that configuration.
What you could do is setup that additional mailbox as a distinct mailbox in Outlook, so rather than being added as an additional mailbox it's a separate mailbox listed in your Outlook accounts. Add it like you would with any other email account,
but you'd use your existing mailboxes login details to authenticate to it. Then and only then can you choose that second mailbox to be the default.
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Marked as answer by
Shlapenka
Sunday, July 19, 2015 3:01 PM