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Currently our email is hosted by a web hosting company and is pop3. I would like to install microsoft exchange on our local server. We have windows server 2008 R2 installed and we already use active directory etc.

The only problem is that we get the odd power cut. The nature of our business means that we cannot afford to miss emails. If the power goes out and our server goes offline, what happens when people are trying to send us emails, do they get stored on another server somewhere? Or do we need a additional backup server somewhere else that comes online when ours is offline?

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up vote 4 down vote accepted

SMTP was designed in an era of the Internet when connectivity was a lot worse, mailers were on the end of part-time network connections, or may only come up a few hours a day. As a result, it's a store-and-forward system.

What this all means is that all (legitimate) mailers will queue up mail going somewhere that isn't receiving. At some point they'll drop the message but exceedingly few of them, and none of the major email services, do so in under 4 hours. At some point, most mailers will deliver what's known as a Delivery Status Notice that the delivery is delayed but being retried. And again if the mail simply can't deliver it and gives up.

Email that is simply lost is generally eaten by grues spam systems, which is a problem you have all the time anyway.

The queued delivery thing throws people off. In olden times, the retry time could be as long as 7 days. These days it's very rare to be under 2 days. Unless it's a natural-disaster inspired power-outage, you'll be up before then. And then over the course of 8ish hours all of that backed up email will land in a flood.

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+1 for the grues - it's nearly always their fault :) – Robin Gill Mar 4 '12 at 3:30
Brilliant, I had no idea this was the case these days. Thank you. – Reafidy Mar 4 '12 at 4:21

In general, when the sending server cannot connect to the recipient server the sending server will queue the email for delivery later and will retry the delivery at preconfigured intervals. The configured intervals are specific to the sending server (the particular MTA is use) and the "drop dead" time (when the sending server stops retrying and sends an NDR to the sender) is usually 48 hours.

It's pretty safe to say that if your server goes down or is unreachable for less than 48 hours that you won't lose any emails. Emails will be delayed in their receipt (depending on the amount of downtime) but they should come in as soon as the server is up and reachable.

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+1 thanks also for your help. Id like to hope 48 hours is more than enough to get the power back online. – Reafidy Mar 4 '12 at 4:23

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