[Dovecot] dovecot OK for intranet, not OK for external IPs

[Gedalya]

On 08/24/2013 05:20 PM, Sulla wrote:
>
> My router forwards the connection request to my mailserver correctly, so
> port forwarding on the router is ok, as my mailserver gets a connection
> request from the client. Proof: "netstat -nt" shows me:
>
> Proto Recv-Q Send-Q Local Address  Foreign Address     State
> tcp        0      0 10.0.0.1:143   10.196.124.7:1024   SYN_RECV
>
>
This proves one thing: that the SYN was received. It doesn't prove that 
the reply is being routed back properly. Obviously you have an 
asymmetrical routing or firewalling issue, and I'm not sure what kind of 
address translation should be / is / is not being preformed.
This is a basic networking issue and has nothing to do specifically with 
dovecot.

Try to shut dovecot down and open a listening port with netcat:

#$ nc -lp 143
or for verbose output
# nc -vvlp 143

On the other LAN compuer:
# nc -vv 10.0.0.1 143

and start "chatting". It will work. From the outside computer it won't. 
If it does, dovecot will work too.
Proceed with troubleshooting. TCP is handled by the kernel, not by dovecot.
(you might want to look at the nc man page for the exact options 
available for your specific variety of netcat)
You can probably do this on any other arbitrary port, it doesn't have to 
be 143 so you can let dovecot run at least for the LAN, if needed, while 
you figure this out.