I am sorry but it seems that I missed your post.
We have been using several providers and settled right now on Postmark where have opened individual accounts for our clients to login and see there statistics, details etc..
But what we are really looking for is a white-label or reseller option.
Within Mailgun, do you have sub-accounts or are just sending all emails through one account?
Can your clients login and see only their statistics and details?
All the best,