andras galos djdpsonetinformhu wietse venema wrote andras galos wietse venema wrote evidence hanging without reply would result different remote smtp client behavior sending 421 reply hanging im rather afraid 421 means try hangup says bad mx try another one evidence actual client behavior thats speculation yes reject everybody 4xx fake mx sender see error message one primary mx case error report delay report could something like 421 mx unavalaible similar message appear senders mailboxes cases im afraid answer client receives something could misunderstood client hangup answer need answer fake mx answer unnecessary point failure really say 4xx ill best regards andras galos wed 06 aug 2008 044229 0100 postfix fake mx