From: Sebastian Ries (Sebastian.Riesdtnet.de)
Date: Fri Apr 04 2008 - 04:37:32 CDT

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Hi

> > If there is an A record for host.example.tld but no MX
> > records for host.example.tld, then mail for
> > userhost.example.tld will be delivered to host.example.tld.
>
> And if host.example.tld is not allowed to recieve mail (by a firewall)
> mail will also be undeliverable. right?

Yes but with different behavior:
- no DNS-Record: "fatal error" -> mail-delivery is ended
  -> sender gets a notification
- no connection (connection timeout or refused) is considered as
  a temporary error -> mail-delivery is retried
  -> then it depends on the configuration of the sending mailserver
     how long it tries to resend the message (mostly 5 days)
     After this period the mailserver generates an error and sends
     the error to the sender
     (mostly a warning is sent after some hours or a day)

Regards
Sebastian Ries

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