Keywords: switchchannel, allowswitchchannel, noswitchchannel. See also saslswitchchannel on 12.4.4 SMTP Authentication, SASL, and TLS, and tlsswitchchannel on 12.4.8 Transport Layer Security and userswitchchannel on 18.104.22.168 Source Channel Switching Based on User or Domain Settings
When the MTA 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 SMTP over 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 initial channel the server uses, the IP address of the connecting (originating) host will be matched against the channel table and if it matches the source channel will change accordingly. If no IP address match is found or if a match is found that matches the original default incoming channel, the MTA may optionally try matching using the host name found by doing a DNS reverse lookup. The source channel may change to any channel marked switchchannel or allowswitchchannel (the default). The noswitchchannel keyword 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.