If you only want SmarterMail to accept SMTP traffic over port 25 for a specific IP you can accomplish this by taking the following steps:

Log in to SmarterMail as the System Admin
Go to Security | Black List
Add the range of IP address you do not want to accept for example if your Gateway is coming from 10.10.10.15 you will black list 0.0.0.0-10.10.10.14 and 10.10.10.16-255.255.255.255.

Now all SMTP traffic from port 25 will be blocked with the exception of your gateway.  If your customers are using mail clients such as Outlook, Thunderbird, etc... you will need to make sure you have an alternate SMTP port enabled or they will not be able to send email.
 
To do this follow the steps below:

Log in to SmarterMail as the System Admin
Go to Settings | Binding | IP Address | (edit IP(s) address) | Enable Submission Port
Click Save.

Now your customers can use an email client to send though SmarterMail, but all port 25 SMTP traffic will only be accepted from the specified IP address.  This way if spammer tries to connect using a different IP address, SmarterMail will reject it.