If you assign a certain IP to a domain in SmarterMail, but mail is still being sent from the main IP assigned to the server's NIC, do the following:

For Smartermail 15x and below:

Log into your SmarterMail Admin Interface
Go to Settings > Protocol Settings > SMTP Out tab
In the Outbound IPv4 area, pop down the box and choose "Use the Domains IP"
Click Save

For SmarterMail 16x and 17x:

Log into your SmarterMail Admin Interface
Click on the Settings sprocket
Click Protocols
In the SMTP Out area under Outbound IPv4, pop down the box and choose "Use the Domains IP"
Click Save