ISA 2006 publishing Exchange

If I have an Exchange server and it is temporarily down, any SMTP server trying to send mail to it will not be able to connect and will hence do retries. What will happen if I publish this Exchange server through an ISA and the Exchange is taken down for maintenance? Will ISA inform the SMTP servers trying to send mail to the Exchange server that it is down so they will retry?
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peakpeakAsked:
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peakpeakConnect With a Mentor Author Commented:
I'm pretty good at mail server communication, just unsure how the ISA will react in a scenario I've described above
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Nyah247Commented:
I don't believe ISA will actually "inform" the servers attempting to connect.  The SMTP connection will just simply fail and the connecting servers will re-attempt according to their configuration.
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peakpeakAuthor Commented:
Ok, what I'm aiming at is that no mail will be lost if the Exchange server is down but ISA is handling the connection. Your reasoning is in line with mine but I'd like a more precise answer.
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Nyah247Commented:
Mail will be eventually lost if Exchange is down for a prolonged period of time.  ISA does receive and hold the email until Exchange is back online.  

Typically connecting mail servers will send a delay message to the sender then re-attempt a connection after a set time.  If it still cannot connect after the configured time (usually 24-48 hrs) the sending server will discard the message and send an NDR.
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peakpeakAuthor Commented:
You mean that the ISA has a mail queue? I don't believe that
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Nyah247Commented:
If you want true failover you will have to setup an additional mail server then configure multiple MX records with different priorities.
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Nyah247Commented:
No...ISA doesn't have a queue.
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peakpeakAuthor Commented:
How can it then "hold the mail" as you stated?
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Nyah247Commented:
That was a typo...sorry.  
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Nyah247Commented:
It cannot hold the mail...  You are "dead in the water" if Exchange is down.
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Keith AlabasterEnterprise ArchitectCommented:
Hey Peter, have got your mail but have only just returned (today) from holiday after a few weeks in sunny Mexico. Didn't ignore you :).

Regards

Keith
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