[exim] Specifying a backup smart host?

I'd like to be able to specify a primary smart host and a backup smart host
in case the primary is down. Is there a way to do this in Exim? Here is my
smart host setup:

dc_eximconfig_configtype='smarthost'
dc_other_hostnames=''
dc_local_interfaces='127.0.0.1'
dc_readhost='your.server.name'
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost='smtp.mailchannels.net::25'
CFILEMODE='644'
dc_use_split_config='true'
dc_hide_mailname='false'
dc_mailname_in_oh='true'

Thanks,
Ken