Hi guys

first the good news:

the tunneling works - just exactly as in any regular linux box

on N900 - online via 3G !!
in
xterm > root > bash

then one commandline to open the tunnel

ssh -L 44999:127.0.0.1:25 root@my.mailserver.IP.xxx

in the N900 email configuration as smtp server:
127.0.0.1

port:
44999

of course you may change port - the one i use since years is a free one on my servers.

the bad news:

I have NO idea why it failed to work at the very beginning.
the only guess I have is the major maemo5 upgrade ( some 112 MB ) may have fixed a system bug ...

the certification stuff is of course already done for all the ssh connection and connection alias I use on my server systems.
on N900 simply use
ssh-keygen
to create your N900 certificate then proceed as usual in Linux environment