Which addon, specifically? I use Outlook Track-It, which is Outlook's main followup e-mail reminder. On this one, you can flag e-mails for a reply reminder, either for yourself or the recipient, or both. It's a great GTD addin.
The GTD Forums are community forums, and thus, open for all to share about what is useful for them in their experience and implementation with GTD.
I would just suggest that you be a little more forthcoming if you work for Outlook Track It, so people know that's why you're suggesting it. (Unless my interpretation is wrong and you are just a huge fan of this software such that in 5 out of your 5 posts on these Forums you have mentioned this software.)
Again, it's fine to suggest your product on these Forums and our blogs, but please do let the the GTD community know the intent in your post if you are trying to sell them something.