If you're used to separating email recipients with commas, Outlook may have trouble understanding you. Usually, it thinks a comma separates a name's last from its first name; she@example.com, Mark Fortunately, you can tell Outlook to treat commas as separators of email addresses, not names.
To have Outlook see commas as separating multiple email recipients:
To make Outlook 2003 and Outlook 2007 recognize commas as separating multiple recipients in an email you compose:
Make Outlook Allow Commas to Separate Multiple Email Recipients
Make Outlook 2003 and 2007 Allow Commas to Separate Multiple Email Recipients

