Exchange - Backup SMTP / MX Server

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Exchange
Server Version
Compatibility 5.5* No

2000 Yes

2003 Yes

2007 Maybe

* May Adapt (More Information)

Backup SMTP / MX Server
Author: Simon Butler, Exchange MVP, MCSE
Last Page Review: 27/12/2006

Exchange can be configured to act as a backup MX server for another domain. Your Exchange server would accept email for another domain whose server is down, and then it would send the email on when the server is back in operation.

This will avoid anyone getting delivery failure messages. However you cannot extract the emails from the queue using this method, as it is just a storage point.

There are is a significant caveat involved, which you should be aware of which is outlined at the end of the article.
Also included is a variant which can work around this problem, and you may want to use that variant as well.

Requirements

Ideally for this to work you will need a second public IP address. This will allow you to create a second virtual SMTP server which can have different timeouts from your main configuration.

External Network (Internet) Configuration

For the domain that you want to be the backup, configure the MX records with a higher value than the primary. So if your primary MX is 5, then configure this server 10 or a higher number.
If you are using a second IP address then remember to use the correct one when creating the host for the MX record.

Internal Network Configuration

Make sure that you don't have this domain listed in your internal DNS information.

Exchange Configuration

There are two parts to the Exchange Configuration. If you do not have a second public IP address, then you can skip creating the SMTP virtual server, and use the default one. However you are then limited to using the same setting that you use for your own email.

Create a new SMTP virtual server.

Open ESM, admin groups, <your admin group>, Servers, <your server>, Protocols.
Right click on the SMTP protocol and choose New, then SMTP Virtual Server.
Give it a name and assign it an IP address.
Once it has been created, right click on it and choose Properties.
Adjust the relay options (click on Relay on the "Access" tab) to ensure that "Only the List Below" is enabled and the list is blank and "Allow all computers which successfully authenticate, regardless of the list above" is deselected.
Apply/OK from that screen back to the main properties.
Click on the "Delivery" tab. Increase the timeouts as required.
NOTE: The retry time can be made quite long. However if the server has been down, then shorten them to get the email flushed quicker.
Apply/OK out and back to ESM.

Create a new SMTP Connector in Exchange.

Open ESM, Connectors.
Right click on Connectors and choose New, SMTP Connector.
Give the connector a name that identifies it as the relay for this domain.
On the General tab, change the option to "Forward all email through this connector to the following smart hosts" and enter the IP Address or DNS name of the other server in the box for Smart Host. If entering an IP address it needs to be enclosed in [ ]: [192.168.1.1]
Under "Local Bridgeheads" add your Exchange server, but choose the alternative SMTP virtual server that you created above.
On the "Address Space" tab, remove the default * and enter the domain name that you wish to store email for.
Enable the option "Allow messages to be relayed to these domains"
All other options can be left as default.

That should be it. Exchange will accept the email for that server, if it cannot connect to the smart host (the real email server) then the messages will queue. This will be a single queue in ESM.
Should someone use the higher value MX record then the email will be delivered immediately - but through the other server.

Caveats

This procedure does bring up a significant problem.
If you are using recipient filtering on the primary server, then email messages will stack up in the queues on the secondary server. Use the variation method below.

Variation

With the caveat outlined above, you may decide that this is not the right thing for your company. However you still like the idea of having a backup mail service. In that case, carry out the follow instead of, or in addition to the above procedure.

Get a backup MX service configured for your domain or domains. This can be another Exchange server as outlined above, with your ISP or another third party supplier.
Create a dynamic DNS address with one of the free service providers that offer it and point it at the same IP address as your existing MX record.
Configure this dynamic DNS host as an additional MX record. This means you have two MX records, with different hosts, but pointing at the same IP address.
All email flows in to your server directly. This lets you do any recipient filtering without upsetting the ISP.

In the event of a failure, get on the Internet using a modem, go to the dynamic DNS provider and switch the IP address of the dynamic DNS host to the email server used for the backup MX service. That server will then start receiving email and queuing it for you. When your server comes back online, simply switch back.

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Last Page Update:
27/12/2006

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