It seems that this important issue is not being addressed at all in SM16!!!!
Let me give you an example of its importance:
We have several Country Clubs with 10's of thousands of Members.
The Clubs send out several Newsletters per year which have to be, by law, registered as received or not.
We therefore use something like Sendgrid which the Clubs pay for.
Each Club has its own Sendgrid account where he only sees his own statistics.
With the current setup we are limited to 10 Gateways (1-10)!
I therefore suggest the usage of a Drop-down where we select the preferred Gateway for that domain.
Hope this helps,