When an IMTA server accepts an incoming connection from a remote system it must choose a channel with which to associate the connection. Normally this decision is based on the transfer used; for example, an incoming TCP/IP connection is automatically associated with the tcp_local channel.
This convention breaks down, however, when multiple outgoing channels with different characteristics are used to handle different systems over the same transfer. When this happens, incoming connections are not associated with the same channel as outgoing connections, and the result is that the corresponding channel characteristics are not associated with the remote system.
The switchchannel keyword provides a way to eliminate this difficulty. If switchchannel is specified on the server's initial channel (tcp_local), the name of the originating host is matched against the channel table; if it matches, the source channel changes accordingly. The source channel may change to any channel marked switchchannel or allowswitchchannel (the default). noswitchchannel specifies that no channel switching should be done to or from the channel.
Specification of switchchannel on anything other than a channel that a server associates with by default has no effect. At present, switchchannel only affects SMTP channels, but there are actually no other channels where switchchannel would be reasonable.
Note - When the switchchannel is specified, the name of the originating host is obtained by a DNS reverse lookup translation of the IP address to hostname. Consequently, this keyword is useful for setting up anti-spamming but it may affect performance.