/etc/postfix/master.cf:
thrttl unix - - n - - smtp -o syslog_name=smtp-throttled
/etc/postfix/main.cf:
thrttl_destination_rate_delay = 10s
thrttl_destination_concurrency_limit = 1
thrttl_destination_recipient_limit = 2
thrttl_initial_destination_concurrency=1
transport_maps = regexp:/etc/postfix/transport
/etc/postfix/transport:
/\@mydomain\.com$/ thrttl: