64K for large table format PSTs (OL 97-2002). If working with offline
folders (ost), the limit is 16K. See http://support.microsoft.com/?id=208480 . I don't know about OL 2003's
Unicode format.
Upon rereading this, are you actually speaking of a Distribution List? Your
ISP's limit should come into play long before Outlook's limit. I suspect
that the Outlook limit is, like items in a folder, either 16k or 64K.