Thanx to this very supportive list!!
Richard pointed out to me that the IP which I called "external" was a net-10 IP and thus no official address, but rather belonged to the private pool of my ISP and that net-10 adresses were not routable with arbitrary results when a router nevertheless tries to route them.
This hint was right on!!! I suspect that my ISP directly routed my connection request back to me without ever unleashing it to the open internet. It's a bit weird.
I just set-up my (ancient) mobile phone (Sony Ericsson z770i) (IP: 213.162.68.141) to connect to my mailserver (I have different network provider, so this MUST go over the internet now), and this works!
netstat -np tells me: Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 10.0.0.1:143 213.162.68.141:29650 ESTABLISHED
and /var/log/mail.log shows: Aug 25 00:01:26 SERVERNA;E dovecot: imap-login: Login: user=<UID>, method=PLAIN, rip=213.162.68.141, lip=10.0.0.1, mpid=24703, TLS, session=<g6kmqrjk0gDVokSN>
and my phone is just synching with the account... Haven't tried SMTP-ing yet...
(it only cost me about 3 hours and a few hairs...)
also thanx to Javier and Gedalya:
The ubuntu 13.04 server installation does not have a firewall enabled (I guess this is typical for ubuntu), at least "sudo iptables -L -n" shows for INPUT, FORWARD and OUTPUT: policy ACCEPT.
route -n shows: Destination Gateway Genmask Flags Metric Ref Use Iface 0.0.0.0 10.0.0.138 0.0.0.0 UG 0 0 0 eth0 10.0.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
which is ok, I guess.
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