Classic Outlook
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"

New Outlook
To ensure this option is enabled perform the following steps:
1. Click on File, then Settings.
2. Select Mail in the left hand pane.
3. Select Compose in the next pane.
4. Scroll down to Recipient separators.
5. Ensure that "Also use commas as recipient separators" is checked.
