"An unexpected error has occurred" or "Internal Support Function Returned An Error" message appears when I try to forward messages. I notice this happens when I use a certain distribution list.
Here are a few suggestions to resolve this issue.
1) You can try updating that distribution list by opening your Address book that it is in and click on the "Update" button.
2) If that does not resolve the issue you will need to delete the distribution list from the Autocomplete (this is when the address is in the memory): Start to type the distribution list name and when you see it highlighted press the delete key on your keyboard.
3) Next, go to your address book and select the distribution list to add it in your email your sending.
4) If this does not resolve the problem then the distribution list may have become corrupt and you may need to delete it and recreate a new one.