If you truly need to manipulate this on mac and windows, you're probably better off using a net-based service (e.g. Constant Contact, MailChimp, others).
Microsoft markets Entourage as its Outlook for Mac, but it's not. It's considerably weaker than Outlook. Don't expect any add-on available for Outlook to be available for Entourage. Don't expect any useful add-ons to be available for Entourage at all.
Any other desktop tools for managing mailing lists will almost certainly not work with mailing tools on the other platform.
Finally, if your list of recipients is large (> 100), you'll need to worry about being flagged as a spammer. The third party services will manage all this for you.
(I do not work for any of these companies, but companies I've worked for have used services like these.)"
A Mac-specific option is Email Merge. It can import data from a text file. You could possibly share this text file between your Mac and Windows machines.
