This requires a new configuration parameter to always defer if the
bind address is unavailable. There is plemnty of code to copy from.
On the oher hand, insead of using a tunnel and juggling routes, you
could use TLS and AUTH for mail that needs to be sent through B.
That code will defer if B is unavailable.
# Use [] to suppress MX lookups.
relayhost = [host.example.com]
# owner root, mode 0600
smtp_sasl_password_maps = username/password for B
transport_maps = overrides for direct delivery
or some sender_dependent configuration.
Wietse
