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By default, Outlook for Windows uses semi-colons to separate a list of e-mail addresses. There is an option to allow for commas as e-mail address separators as well.
To enable this option, perform the following steps:
1. Click on File, then Options
2. Select Mail in the left hand pane
3. Scroll down to the "Send messages"area
4. Check "Commas can be used to separate multiple message recipients"