Navigate to KB0011408 - Unblocking Your Email Server for a description of the Talos reputation service.

Neither new nor extremely low-volume email servers have Talos reputation scores. Unfortunately, newly activated spam zombies (compromised devices being used to send spam) also do not have reputation scores, since they usually never send email at all before being activated. For these reasons, hosts with no reputation scores are considered somewhat suspicious, and are subjected to a rate limit of 10 recipients per hour.

If the system administrator of a legitimate new or low-volume mail server wishes that mail server to be exempt from this limitation, they should contact postmaster@utexas.edu to resolve the problem. Requests must include the IP address(es) of affected server(s), as well as the date and approximate time (including time zone) of the most recent incident.