A couple of days ago, someone (I've discarded the original, sorry) asked why subscription requests could arrive from a user at SPARCY.MOBITEL.TELIA.SE but all attempts to reply to that host bounced. I didn't see an answer here, so I posted the question to [log in to unmask] and got the following reply: On Tue, 12 Jul 94 13:54:19 -0400, William H. Magill <[log in to unmask]> wrote: >> >Subject: unknown node: SPARCY.MOBITEL.TELIA.SE >> > > >The reason is actually simple and mentioned above - SPARCY.MOBITEL.TELIA.SE >is not registered in the DNS and has no MX record > > MOBITEL.TELIA.SE >is a subdomin > > SPARCY >smells like > sparc-Y >implying a SUN sparcstation sitting on the backside of a SUN-YP, >double-net, configuration. In this case only the server is registered on >the DNS, all of the clients on the backside (a separate network) send their >outgoing traffic through the server. > >It is not unusual for such environments to be incorrectly configured so as >to allow the backside clients to run sendmail and send mail DIRECTLY to the >outside world with their own hostname as a reply address. In reality, >however, only the "gateway" SUN is configured to be visible. > >So try sending to: > > [log in to unmask] > > >>220 oasen.mobitel.telia.se Sendmail 5.65/DEC-Ultrix/4.3 ready at Tue, 12 Jul >>199 >>4 19:40:40 GMT >>vrfy staffan >>250 staffan... Staffan Hellenberg <staffan> >>quit >>221 oasen.mobitel.telia.se closing connection >>Connection closed by foreign host. >> > >I just worked backwards with nslookup, and picked on the "mail addr" host.