Throttling outgoing mail really helps with big ISP like google or (here in Germany) United Internet or T-Online. Right now we have to use a Postfix server as smarthost to establish this.

Postfix documentation:

smtpdestinationconcurrency_limit: "The maximal number of parallel deliveries to the same destination via the smtp message delivery transport."

smtpdestinationrate_delay: "The default amount of delay that is inserted between individual deliveries over the same message delivery transport, regardless of destination".

smtpextrarecipient_limit: "This extra recipient space is reserved for the cases when the Postfix queue manager's scheduler preempts one message with another and suddenly needs some extra recipients slots for the chosen message in order to avoid performance degradation.".