Um zwei SMTP-Server (z.B. Mailserver im Heim-Netz zu Mailgateway bei Provider/Hoster) sicher miteinander zu verbinden, gibt es verschiedene Möglichkeiten.

Eine Möglichkeit ist, ein OpenVPN-Tunnel zwischen beiden Servern.

Eine andere Möglichkeit ist ein SMTP-over-SSH-Tunnel:

/usr/local/bin/smtp-tunnel.pl

#!/usr/bin/perl
 
# A SMTP-over-SSH Port Forwarding Scrpt
# Copyright (c) 2010 Linode, LLC
# Author: Philip C. Paradis <pparadis@linode.com>
# Modifications: Linode <sam@linode.com>
# Usage: smtp-tunnel [start|stop]
# Forward smtp traffic over ssh to a remote mailserver.
 
## Edit these values to reflect the authentication credentials for the
## SMTP server with which you wish to connect and send mail. If you
## have chosen to run your mailserver on your linode using an
## alternate port, modify the `$remote_port` value. You should not
## need to modify the `$remote_ip` value.
 
$remote_user = "REMOTE-USER";
$remote_host = "REMOTE-HOST";
$remote_port = "25";
$remote_ip   = "127.0.0.1";
 
## Modify these values if you are running a local SMTP server on port
## 25, or if you need to start the tunnel as a non-root user, as
## OpenSSH only allows root users to start tunnels to low-numbered
## "privileged ports." If this is the case you will also need to
## modify the configuration of your local mail sending agent. 
 
$local_ip    = "127.0.0.1";
$local_port  = "25";
 
## You do not need to edit this file beyond this point. 
 
######################################################################
 
$a = shift;
$a =~ s/^\s+//;
$a =~ s/\s+$//;
 
$pid=`ps ax|grep ssh|grep $local_port|grep $remote_port`;
$pid =~ s/^\s+//;
@pids = split(/\n/,$pid);
foreach $pid (@pids)
{
 if ($pid =~ /ps ax/) { next; }
 split(/ /,$pid);
}
 
if (lc($a) eq "start")
{
 if ($_[0]) { print "smtp-tunnel already running.\n"; exit 1; }
 else
 {
  system "ssh -f -L $local_ip:$local_port:$remote_ip:$remote_port $remote_user\@$remote_host -N";
  exit 0;
 }
}
elsif (lc($a) eq "stop")
{
 if ($_[0]) { kill 9,$_[0]; exit 0; }
 else { exit 1; }
}
else
{
 print "Usage: smtp-tunnel [start|stop]\n";
 exit 1;
}

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