Hi @Christian Kl ,
As I know, there is no option in Outlook to force it to always reply with the email address of your default account, which is by design and we couldn't change it, for more information, please refer to this document : Change the account used to send email messages.
As a suggestion, vba script may have some effects in theory , due to here we mainly focus on general issues on Outlook desktop client and know little about Outlook. If you are familar with vba development, maybe you could try to research your own script and post the issue in relevant development forum when you have development issues.
Hope that would be helpful to you.
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