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 supported on ports 25, 2525, and 587.
To enable IP authentication, navigate to your Account tab and select IP Authentication. From there, click on the +New IP button. A window will open up to enter the IP address you will be connecting from and add an optional description for that IP.
Do you not know what your IP address is? Simply use the marker icon to the right to autodetect your current IP address.