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NAT for SMTP

Hi, guys !

I got a problem with an Exchange mail server - it is located in a private LAN, behind a NAT device (Cisco 1712 - c1700-k9o3sy7-mz.123-7.T1.bin).

The problem I have is that I cannot connect using SMTP from outside - Outlook Express reports various errors.

After doing some investigations I found out:

- telnet on the real (private) IP address of the email server on port 25 works fine

- if I try it for the public IP address of the router which is translated for port 25 to the internal IP of the exchange server port 25, then I get the login banner from the Exchange server, but immediately after I type the first character in this session I get "Connection to host lost." .

I removed all access lists and the symptom persists. No CBAC configured.

The NAT statement is

ip nat inside source static tcp <internal IP> 25 <external IP> 25

Similar statement for POP3 on port 110 - and telnet works fine on tcp:110.

Does anyone have any ideea here ?

I would suspect a NAT configuration problem, like wrong timers or something related to the translation properties but I am really lost here.

Any suggestion would be appreciated.

Thanks

2 REPLIES
VIP Purple

Re: NAT for SMTP

Hello,

can you try and add the keyword ´extendable´ to your static NAT translation statement:

ip nat inside source static tcp 25 25 extendable

and check if that makes a difference ?

Regards,

GP

New Member

Re: NAT for SMTP

Hi, GP

"extendable" is added by default after I enter the NAT command.

The show run displays the command as you suggest it.

BR,

Dragos

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