The destination_concurrency_limit and rate_delay parameters can be configured per transport. You could configure a transport map to route all mail destined to domain.com via the example: transport, which can be configured as a clone of the smtp transport in master.cf. Then, specify concurrency and other limits/delays for only that transport in main.cf.
This way? /etc/postfix/master.cf: slow inet n - - - - smtpd /etc/postfix/transports: domain1.com slow: domain2.com slow: /etc/postfix/main.cf: slow_destination_concurrency_limit = 1 slow_destination_rate_delay = 1 If I understood it correctly, it should be something like that, isn't it? -- Santiago Romero
