Many major ISP's (such as Hotmail, AOL, and Yahoo) will block large email campaigns that flood mail servers with too many incoming connections. XM Mail Server features a smart deliverability algorithm called slow start outbound connection ramping.
The server keeps track of all the domains and IP addresses that it has communicated with in its current session. It also keeps track of the failures that specific domains have reported. For instance if hotmail.com is returning 400 level status codes (temporary failures).
The slow start outbound connection ramping algorithm restricts the number of outbound connections to a domain based on this success/failure rates of the domain. If a domain is not blocking your email, the algorithm will slowly increase the number of connections to the domain. If the connections to the domain are failing the algorithm will ramp off its connections.