new SMTP server testing

[m.chandra2010]

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 192.168.1.50 25 interface FastEthernet4 25

and the new server IP will be 192.168.1.100

Thanks

[Collin Clark]

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

[m.chandra2010]

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 192.168.1.100 25 interface FastEthernet4 25230

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

Thanks

[Collin Clark]

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.