_destination_concurrency_limit precisa ser > 1.
O valor 1 fará com que o destino seja e-mail completo, valor > 1 fará com que o destino corresponda ao domínio de e-mail. No entanto, o destination_rate_relay quando definido sempre será enviado um de cada vez.
default_destination_concurrency_limit (default: 20)
The default maximal number of parallel deliveries to the same destination. This is the default limit for delivery via the lmtp(8), pipe(8), smtp(8) and virtual(8) delivery agents. With per-destination recipient limit > 1, a destination is a domain, otherwise it is a recipient.
tente
smtp_destination_recipient_limit = 2
smtp_destination_concurrency_limit = 2
smtp_destination_rate_delay = 1s