All paid plans come with the option to enable IP authentication. This allows SMTP connections from a given IP address without requiring authentication. Any SMTP connections from a configured IP address will automatically log in as this user. IP authentication is support on port 25, 2525, and 587.
To enable IP authentication navigate to your Account tab and then select IP Authentication. From there, simply click on the +New IP button. A window will open up where you can enter your IP address and add a optional description for that IP. Do you not know what your IP address is? Simply use the marker icon to the right to auto detect your current IP address.