Incoming channel match and switch channel options

Many MTA configuration choices look at the incoming channel (source channel) for a message. A number of channel options affect such initial channel "matching" and channel "switching".

See also:
 * allowswitchchannel Option
 * nosaslswitchchannel Option
 * noswitchchannel Option
 * saslswitchchannel Option
 * switchchannel Option
 * tlsswitchchannel Option
 * userswitchchannel Option
 * Functional group list of channel options