This is probably more of a general UNIX/network question, but it hits me in connection with kmail.
I am using kmail (locally) at work to send mail via exchange/smtp server. This is debian/testing, thus kmail 1.13.7. Sometimes my local email winds up in the junk mail folder of my colleagues. So trying to identify the problem, our sysadmin pointed out that the Received: from envelope field contains a strange domainname (localnet). My email address is ok in all cases, outgoing/incoming/reply, just the system identification towards smtp is funny.

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Received: from nbofm020.localnet (10.82.2.244) by
  exchange.ipmlan.ipm.fraunhofer.de (153.96.204.14) with Microsoft SMTP Server
  (TLS) id 14.1.438.0; Fri, 22 Feb 2013 15:59:28 +0100
 To: My Name <myname@ipm.fraunhofer.de>
 Subject: ...

I wonder where kmail gets this information from. On my computer, domainname returns nothing, the resolv.conf
looks like

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# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 153.96.204.4
search ipmlan.ipm.fraunhofer.de

and /etc/hosts is

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127.0.0.1       nbofm020        localhost.localdomain   localhost
192.168.2.10    marmotte.local marmotte

# The following lines are desirable for IPv6 capable hosts
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters

If anybody could give me a pointer to where the information comes from I could probably fix it...
Thanks in advance.

Can you check under KMail configuration > Accounts > Sending? There should be a line edit at the bottom with the domain. Check if it's set.

I assume that your refer to the field 'Default Domain' in 'Setup for Sending and Receving Messages' which one reaches in the way you describe: that field is set to 'ipm.fraunhofer.de'.

Triggered by your comment, I checked again another option that can be reached from that dialog via Sending > Modify > Advanced. There are
'send custom hostname to server' / 'use custom sender address' check boxes and fields. I had both are unchecked and empty. If I fill the hostname field with a FQDN (nbofm020.ipm...), this gets used in the 'Received: from' when communicatin with the smtp server.

It doesn't answer the question where the 'localnet' comes from, but it's a reasonable workaround.