Outlook has a setting to control whether or not a comma can be used as an address separator (the default separator is a semi-colon). It's disabled by default in Outlook (Tools | Options | E-Mail Options | Advanced E-Mail Options), although the same setting in Outlook Express is enabled by default. Personally, when I install Outlook, checking this box is one of the first things I do.

A few years ago we investigated turning this setting on by default. We were unable to do so, because many organizations use the comma in their naming scheme in the directory - "Lastname, Firstname". If we enabled comma as an address separator by default, Outlook would interpret me typing in "Lemson, KC" as two separate addresses, and would try to resolve "Lemson" and "KC" separately. This helps explain why OE has it enabled by default - with OE, you only have to worry about internet e-mail and corporate directories are not as common of a concern.