I know with exim it's possible with some settings.
You can visit
http://www.exim.org
exim.conf
# Incoming tweaks
#
# Max number of simultaneous SMTP calls to accept
# (aka max number of exim processes) Default is 20
smtp_accept_max = 5
# Max number of waiting SMTP connections.
# Gives some protection against denial-of-service attacks by SYN flooding
# Default is 20
smtp_connect_backlog = 5
# Max number of MAIL commands that Exim is prepared to accept over a
# single SMTP connection, after which a 421 is given. Default is 1000
smtp_accept_max_per_connection = 50
# Max number of simultaneous incoming SMTP calls before messages
# are just placed on the queue. Default is 0 (no limit)
smtp_accept_queue = 150
# Max number of delivery processes that Exim starts automatically when
# receiving messages via SMTP before starting to queue. Default is 10
#smtp_accept_queue_per_connection = 15
# When smtp_accept_max is set greater than zero, this option specifies a
# number of SMTP connections that are reserved for connections from the
# hosts that are specified in smtp_reserve_hosts. Default is 0
#smtp_accept_reserve = 0
# Hosts for which SMTP connections are reserved (host list, expanded)
# Default is unset
#smtp_reserve_hosts =
# Max system load before not accepting SMTP calls,
# except for hosts defined in smtp_reserve_hosts
# Default is unset (fixed point)
#smtp_load_reserve
