Hi Kenny,
It would have been better if you would have provided the error log but you can always do it later. Few questions here:-
1. Are you trying to send this email through a workflow or a plugin?
2. In that case have you changed his security role to send on behalf of other users.

If this doesn't works, please provide the error log.
Regards, Abhishek Bakshi If you find this post helpful then please Vote as Helpful and Mark As Answer. Check my blog on https://mydynamicscrmblog.wordpress.com/