On second consideration, if an *SQL "server_hosts" setting specifies
only one target (host or IP address), then Postfix has little to
lose if it pretends that the name is given twice, and retries once
immediately, especially of it turns off the logic to avoid a failed
*SQL server for 60 seconds.
The logic for LMTP and SMTP can be similar: if a next-hop destination
resolves to exactly one IP address, pretend that it is given twice,
and retry once immediately. If both attempts fail, defer mail as usual.
Bulk mailers may want to skip the immediate SMTP retry if the error
was at the network-level (no connection) because an SMTP client
that is retrying a "down" host cannot be used to deliver mail.
This retry logic does not aply to milters, where a failure in the
middle of an SMTP conversation is not immediately recoverable.
Commands would have to be replayed and message changes would have
to be undone. Instead, the remote SMTP client has to retry the
entire transaction later.
Wietse
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