Not security-related...
The problem seems to be, that all (common) mail user agents create the envelope sender address using the from address.
That would depend on the delivery method. As no envelope sender can be configured in kmail for SMTP delivery method, there's no choice but to use From:. For delivery method "sendmail", the envelope sender is determined by the local MTA and would by default be $USER@<local host name>. You must change this with yast (variable FROM_HEADER in /etc/sysconfig/mail), because no-one will receive mail from you when you call yourself $USER@linux.local, you need to call yourself $USER@$FROM_HEADER, and $FROM_HEADER must be a domain which resolves (e.g. your ISP's, but make sure $USER@$FROM_HEADER isn't someone else's address!!). It is not very difficult to configure a local MTA for sending mail, and you can configure that to deliver directly, or relaying via your ISP. You can specify a delivery program in kmail and use argument -f, if you can't use arguments there use a wrapper script. For sendmail, you'll also have to add yourself to the trusted users list, or sendmail adds a warning header line to each mail. You can also use header rewriting in your local MTA, but postfix's is rather limited and I couldn't get it to do what I wanted, so stayed with sendmail. Volker -- Volker Kuhlmann is possibly list0570 with the domain in header http://volker.dnsalias.net/ Please do not CC list postings to me.