Hi Andy,
Generally, Outlook 2010 will automatically select the sending account based on the currently selected folder. For example, if you click the Inbox folder of your internal work account, the From field would use your internal address by default.
If you want to have Outlook prompt you to select an account when sending a new message, please create and set the forceaccountselection Registry value for Outlook 2010:
Key: HKEY_CURRENT_USER\Software\Policies\Microsoft\office\14.0\outlook\options
Value name: forceaccountselection
Value type: REG_DWORD
Value: 1
Note: This setting only applies to new messages and not to replies and forwards. Replies and forwards will always use the same account that you have received the original message with.
Regards,
Winnie Liang
TechNet Community Support
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