smtpdcheck checks to see if SMTP servers are available. The intent is for use on secondary mail servers, which have no reason to accept mail when the primary server is available.
The argument consists of a list of server names, each of which may optionally be prefixed by a numeric MX priority and a colon. (This is exactly the format for MX records returned by the avenger dns command.) smtpdcheck will attempt to connect to each server in succession.
If one of the servers specified on the command line is available, smtpdcheck will print its name to standard output and exit with status 1. If smtpdcheck cannot connect to any of the servers, it will exit with status 0. If a system error occurs, smtpdcheck will exit with status 2.
If a gethostbyname lookup for the argument name fails, smtpdcheck will exit immediately with status 2.
By default, smtpdcheck spends 10 seconds probing each server. This includes the time to do a DNS lookup, to establish a TCP connection to port 25 of the server, and to read the "220" SMTP code from the server's SMTP greeting message. To use a different value, specify it with the --timeout option. The value 0 disables the timeout completely, which is dangerous since smtpdcheck might then end up waiting forever to read the "220" string.
test -n "$server" && defer "Please use server $server"
smtpdcheck could achieve much lower latency by probing all the servers simultaneously. It should also include some kind of caching, to avoid repeatedly trying to contact an unavailable server. Finally, hosts with multiple IP addresses could be handled more cleanly, though what smtpdcheck does should probably work in most cases.