SMTPCONF="MX:{-MX.*,*}"
which means trying the MX first, and if it failed, then try direct
connection to the domain as a hostname.
Examples:
SMTPCONF="MX:smtpserver"
-- use the smtpserver as the upstream SMTP server
SMTPCONF="MX:{smtpserver1,smtpserver2}"
-- try the SMTP servers in the order
SMTPCONF="MX:{-MX.*,*}"
-- the default configuration
SMTPCONF="MX:{-MX.*,*,smtpserver}"
-- adding a backup SMTP server
SMTPCONF="MX:{*,-MX.*}:*.localdomain"
-- forward directly to the host if it's in localdomain
SMTPCONF="MX:smtpserver:{*.dom1,*dom2}"
-- a server for the specified domains