As these are outgoing emails you would find all available rate limits under Mail Policies -> Mail Flow Policies -> Relay and can modify that based on your requirement. The rate limits are applied based on the sending server and sender email address.
If the rate limit is to be set for certain internal servers and not all you can add a separate mail flow policy with relay action and a separate sendergroup specifically for them.
The explanation for each limit can be viewed by clicking on Help and Support -> Online Help on the top right.
ESA uses a single queue for all emails processed through it, incoming and outgoing.
Regards,
Libin Varghese