One of my NetBSD machines has recently moved to a new network which is
behind a firewall.  It now has no publicly accessible DNS entry.
Although all outgoing network services (e.g., telnet, ftp, ping,
traceroute, http, ssh) work fine, mail delivery no longer does.

I presume that this is because hosts at the receiving end of an smtp
connection do a DNS lookup and then refuse the connection when they
cannot find an entry for my machine.  I presume also that the correct
solution is to obtain a public DNS entry for my machine.

- Is this correct (or are there better solutions)?

- Will obtaining a public DNS entry expose the local network to any
  additional vulnerabilities?

Thanks for your help.

Cheers,
Brook