INTRODUCTION
I've got 1 primary server (Server1), and 2 secondary (Server2 & Server3).
Server3 is made with jail, so additionaly I've 2 more servers (Server3a,
Server3b). All servers are bimaped by Server1 with ipnat as in example:

map vlan1 192.168.1.3/32 -> 80.200.300.400/32 proxy port ftp ftp/tcp
map vlan1 192.168.1.3/32 -> 80.200.300.400/32 portmap tcp/udp auto
bimap vlan15 192.168.1.3/32 -> 80.200.300.400/32

Every server has got its own global IP.

THE PROBLEM
When I want to send an e-mail from Server3a to Server3b the message is
bounced with log:

connect to Server3b [80.200.300.400]: Connection refused)


Trying some telnet connection:

23:23:root@Server3a :/home/kayteck# telnet 80.200.300.400 25
Trying 80.200.300.400...
telnet: connect to address 80.200.300.400: Connection refused
telnet: Unable to connect to remote host

and when I try the same connection but using the local IP

23:23:root@Server3a:/home/kayteck# telnet 192.168.1.3 25
Trying 192.168.1.3...
Connected to 192.168.1.3.
Escape character is '^]'.
220 Server3b ESMTP Postfix

I can only send e-mails from bimaped servers to primary server...

QUESTION
How to bimap or use some other solution to solve this problem - sending
e-mails between secondary servers...

Helpful will be to add that Server1 and Server2 have IP adresses from
different IP class than Server3, 3a and 3b.
Problem is not only with port 25 but also with every other port while
connecting to global IP.

All IPs and hosts are fake - just for better understanding.

Thanks in advance for ever idea