Until version 10, ListManager could only send messages over one IP address, which was fixed for all the lists associated with a particular site. ListManager 10 introduces the concept of mail streams, which bind one or more IP addresses together to form a common sending unit. This sending unit has the ability to cooperatively send particular messages out in parallel. Not only does this help speed up sending into the major ISPs by potentially opening up more parallel connections during delivery, but it greatly increases the tolerance of sending blocks. When a destination server signals (via the SMTP response text) that it will not accept any more email from a given IP address, a "block" is established; ListManager records that block, which consists of a sending IP/destination IP address pair. ListManager stops sending from that IP address to the particular destination IP address, and displays that block in the mail streams report. The block helps prevent further deterioration of the reputation of the IP address.
ListManager will probe the block hourly to see if it is still in force. Server administrators can also intervene and clear blocks once the situation that caused the block has been resolved. During the time that a particular block is in place, ListManager can still send messages out on the other IP addresses associated with the mail stream in order to maximize delivery. If a destination domain is totally blocked (i.e., there are blocks for every possible IP pair between the mail stream and the destination domain), ListManager stops pulling recipients for the blocked domain for messages using that particular mail stream. This maximizes ListManager’s ability to keep sending to other destinations.
Mail streams can have default associations with sites, lists, and segments. Any mail stream available on the server can be used for any message being sent from the server.