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New Member

new SMTP server testing

Hi Guys

can you help me with the following?-

we have an Exchange migration project, so I will need to make changes on the firewall to allow the emails to be delivered on the new server.

having a different IP address on the new server, I will need to create a new NAT rule and access-lists.

the question is if I use port 25 for the new NAT rule it will remove the existing NAT rule which I dont want as the new mail server is still in the testing phase and I dont want to break the connection.

can you please advise how do I test this new server for email sending without removing old one?

I am using a CISCO881 router.

existing NAT rule is-

ip nat inside source static tcp 25 interface FastEthernet4 25

and the new server IP will be


  • Firewalling

new SMTP server testing

Do you have more public IP's than the one assigned to Fa4 or is that the only one?

New Member

new SMTP server testing

Hi Collin

no I have only one Public IP address.

just a question on top of the question-

what about if I create the new NAT rule with a different external port?

i.e.- ip nat inside source static tcp 25 interface FastEthernet4 25230

and then allow the port 25230 on the new server, will that work or I am talking non-sense?


new SMTP server testing

Using a different port will work fine and since you have only 1 public IP, that is the only way to do it without breaking the production NAT for email.

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