[exim] unlimiting message per connection

I have a server that often send many messages in a few seconds , to
address to another server. I say 50 to 1000 in 15" .
I note thet only a few are sent per connection, waiting for another retry
to send more.
I know in advance the pair of server, and the kind of message, so it would
not be a security concern disabling messages limit per connection
How to set exim to tell that server X when sending to server Y should send
all the messages in the queue atr the maximum rate, even if there are
thousands, and to the other serve tho accept such requests, only from a
predefined group of hosts ?