Hi @Tim Metivier ,
I noticed that you seem to would like to reply a message sent to other email account instead of your default gmail account, is it right? If so, as I know, the address used when you reply a message is actually related to what account is received the message instead of the default account settings, which means if you want to Reply, Reply All, or Forward a message, Outlook will send your message with the same account the original mail was sent to, it is by design and there's no option in Outlook could change it.
For more information, please refer to this article : Change the account used to send email messages.
Hope that would be helpful to you.
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