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.