Fortunately, they don't have to know — and you can take them with you. Save your Outlook address book to a comma-separated plain text spread sheet, and OS X's Address Book application (which Mail uses for contacts management) can import them with nary a hiccup.
Just make sure you do import all the email addresses!
Import Your Outlook Contacts into the Mac OS X Mail Address Book
To import Outlook contacts in the OS X Address Book used by Mail:
- Export your Outlook contacts to a CSV file called "ol-contacts.csv".
- Copy the "ol-contacts.csv" file to your Mac.
- Launch Address Book on your Mac.
- Select File | Import | Text file... from the menu.
- Search for and double-click the "ol-contacts.csv" file.
- Make sure Ignore first card is checked.
- Look for E-Mail Address in the ol-contacts.csv column.
- Click Do not import in the Address Book column's corresponding field.
- Select Email (work), Email (home) or Email (other).
- Optionally, map the ol-contacts.csv's Email 2 Address and Email 3 Address as well.
- Click OK.
For drag-and-drop mapping, try Abee.