Lastresort Channel Option

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Specify a last resort host for delivery (lastresort)

The lastresort channel option is used to specify a host to which to connect when all other connection attempts fail. In effect this acts as an MX record of last resort. This is only useful on SMTP channels.

Note that the lastresort host is attempted only for hosts that are in the DNS, having either MX records or an A record, and for whom the connection attempts to all the MX records -- or to the A record, if there were no MX records---have encountered temporary connection failures. (In particular, the lastresort host will not be attempted for a host that is only in the hosts file, not in the DNS at all. Also keep in mind that a permanent 5xx error in response to a connection attempt to a host is a permanent error, and will result in bouncing a message; in particular, the lastresort host will not be attempted after such a permanent rejection error. Also, the lastresort host will not be attempted if a connection succeeds, but the MTA's wait for an SMTP banner line to be returned times out; that again is not a temporary connection failure.)

This channel option requires a single parameter specifying the name of the "system of last resort".

See also the IP_ACCESS mapping table, which can provide an alternate way of doing "fail over" for outbound IP connections for SMTP and LMTP channels.

Note that in most cases, it is preferable to fix problematic DNS records rather than to use lastresoft; lastresort is intended merely for a few, special sorts of cases where correcting DNS records may not be possible, yet some "last ditch", MX-like, re-routing may be useful.