It is a Windows setting.

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Milly Staples [MVP - Outlook]
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After furious head scratching, Hemp Danny asked:
| How do I change the default contact address country/region in Outlook
| Contacts
Dragonscar - 02 Dec 2005 23:09 GMT
Okay, so how does one change it in Windows? It only seems to show up in
"Mailing Address" in Contacts, but then it is spelled out. Is there somewhat
to format "Mailing Address" not to show it? Or at least change it from
"United States of America" to "USA?"
> It is a Windows setting.
>
> | How do I change the default contact address country/region in Outlook
> | Contacts