By default Postfix will use smtp as transport method when none of the sender based rules is matched.
default_transport (default: smtp)
The default mail delivery transport and next-hop destination for
destinations that do not match $mydestination, $inet_interfaces,
$proxy_interfaces, $virtual_alias_domains, $virtual_mailbox_domains,
or $relay_domains. This information can be overruled with the
sender_dependent_default_transport_maps parameter and with the
In order of decreasing precedence, the nexthop destination is taken from
$default_transport, $sender_dependent_relayhost_maps, $relayhost, or
from the recipient domain.
Specify a string of the form transport:nexthop, where transport is the
name of a mail delivery transport defined in master.cf. The
:nexthop destination is optional; its syntax is documented in the
manual page of the corresponding delivery agent.
default_transport = uucp:relayhostname
To override this, as mentioned above, you can set
default_transport parameter directly in main.cf. All the available options you can see under Example in http://www.postfix.org/transport.5.html.
For your sender based routing or transport rule you can put an wildcard entry (*) at the end which will be matched for "all other sender domains that not defined explicitly" and point it to any of your available next hop or something else.