I know that major ISP's like hotmail and yahoo look for how many
connections they get from an ip address.
HOW do you know this ?
Experience by many professional mail admins shows that they throttle senders solely based on the amount of mail they (try to) send in a given timeframe, not the amount of connections used.
One can still send 100 messages per second over one or two connections; the big ESPs will cut you off way before that point.
To answer your questions:
- This is controlled by
default_destination_concurrency_limit, which defaults to 20 connections per destination
- Up to
max_use messages will be sent over one connection, provided there are that many to send.
For a single destination host, this will typically never be reached in one session.
- This involves several steps: first, create a new transport for each destination, and set its $destination_default_concurrency_limit, and next, route traffic to these transport using transport_maps.
You can find a wealth of information concerning such setups if you search the postfix mailinglist for "yahoo slow transport"