sendmail from 4.10-STABLE firewall

[Joe Kraft]

I'm using a 4.10-STABLE based firewall, which is happily chugging along. 
  It's sending it's daily messages to a local account via sendmail, 
which I check by logging in using an ssh connection.

I would like to have it send those mails to another mail server behind 
the firewall, but I'm curious recommendations for the best way to do 
that.  I've looked at a couple of different ways:

1) Add the local domain DNS to it's list so it can find the IP of the 
local mail server.  It seems like this would decrease the security of 
the firewall a bit.

2) The handbook has a section about setting up to send only using the 
mail/ssmtp port.  This looks like my local mailer needs to resolve 
somehow (/etc/hosts??) on my firewall

3) Is there a way to convince sendmail to send to something like 
foo at 10.0.0.55?  I could just put that in my existing aliases file and 
not have to install anything more.

4) Better options??

I appreciate recommendations you can provide or any pointers to existing 
information to help me figure out the best solution.

Thanks,
Joe Kraft