Private domains differ from public domains (such as yahoo.com and gmail.com) because they are exclusive to users who are, in some way, associated with a business or organization. The private domains of most businesses/organizations can be easily identified in email addresses because the domain immediately follows the @ symbol and ends with a domain extension (such as .com, .net, .au, .ca).
The "private" portion of the domain is often the business/organization name or abbreviation, such as higherlogic, as in email@example.com.
Private domains often have more complex email-filtering in place in order to mitigate the possibility of breaches by nefarious agents (malware and ransomware). Breaches can disrupt service and be very costly to businesses/organizations.
Many businesses/organizations protect their email systems by employing IP address "allow" and "block" lists. Email traffic from the senders on the "allow" list passes into the system.
While using an "allow" list increases the likelihood of your messages reaching private domain inboxes, it is not guaranteed. You should still use recommended Communications Enterprise best practices to ensure that your messages have the greatest chance to reach the recipient and to avoid being classified as spam.