Adding the eNotify server IP addresses to your email server's whitelist is a good way to ensure constituents receive messages sent from eNotify. Here is the current list of eNotify server IP addresses from which eNotices are sent:
Finalsite adds new servers as needed and adjusts the servers that schools use in order to balance load inequities that arise. As such, it is best to whitelist the full range of our eNotify IP addresses. It's not prudent to whitelist only the server you currently use, as it is not possible to say your IP will always stay the same.
Refer to the software documentation to confirm the correct syntax required by your spam filter.