Thanks very much Andreas, the Qube now knows its own hostname and 
proftpd is happier (I still have an issue with "scoreboard not found" 
to fix, but that shouldn't be bad- although I'm not sure why the 
install didn't set that up).  I can set up these services on classic 
MacOS with DHCP enabled, but apparently static is needed here.

I still have a problem with sendmail though.  I can send mail out just 
fine, but I cannot receive external mail.  Servers outside the LAN 
can't get my mailserver to respond.  It's not a 550 relaying denied, 
it's just not there.  Port 110 won't respond either.  If I change my 
port forwarding to send ports 25 and 110 to my other, which I have had 
setup for years, all works just fine.  Point them back to the Qube, 
messages cannot arrive or be checked via POP3.

I checked sendmail.conf and added my host name, and also added my 
external domain name to local-host-names.

I've googled a lot and not found anything helpful yet, other than it 
might still be a DNS error of some sort per  some of the RedHat sites.

/var/log/maillog only has logs of my outgoing mail, no note of any 
blocked or failed connect attempts from outside the LAN.

Googling brings up a few posts that indicate that some installations by 
default only accept local mail, but there is no mention of where to 
change this.  And I don't see anything appropriate in the sendmail.conf 
file related to this.  Only the local-host-names specification which I 
understand should only contain the name of the domain for which I 
expect to receive mail (and it does).  delivery-mode is set to 
"background" so things are not being held anywhere.

It's not that I don't have forwarding or my external domain at the 
registrar set up properly; I can ssh into the Qube just fine, either by 
using my registered domain name or WAN IP.

It's got to be a sendmail config issue, likely refusing outside 
connections; but where to set that? I have a RedHat 5 text, went 
through the whole sendmail config file, nothing leaps out besides the 
first options that I already set, mentioned earlier.

I read a bit in a FreeBSD  book while at the bookstore; they said, 
"sendmail is neat and if it works out of the box for you, that's great. 
  If it doesn't, don't bother to try to fix it; just use qmail".

Is it time for Qmail?   But I'm stubborn...

thanks,

Brian