If you keep your own mailing lists in Word or Excel and copy/paste them when mailing groups of people, you may be having trouble if your lists are comma-delimited, e.g.:
user1@potsdam.edu, user2@potsdam.edu, user3@potsdam.edu
Outlook requires either a linefeed or a semicolon between addresses in a list of addresses. So:
user1@potsdam.edu; user2@potsdam.edu; user3@potsdam.edu
or:
user1@potsdam.edu
user2@potsdam.edu
user3@potsdam.edu
Update your lists!