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Although the process for adding new domains is easy enough, warning users who still continue to receive messages via the old domain name can be a bit tricky. But it is certainly a good idea to do this, especially if you don’t plan on retaining the old domain name forever.
I recently had to phase out an old domain name and warn recipients who continued to use the old addresses. Although transport rules are the logical choice for this process, there are a few illogical twists that you need to watch out for when configuring your rules.
First, start the Exchange Management Console (EMC). Then, navigate to the Organization Configuration > Hub Transport > Transport Rules tab. From here, you’ll want to create a new transport rule with a name that makes sense for your scenario.
The interesting twist is encountered when you set up the conditions of the transport rule. You may presume that you would want to create a rule that says “Where the To address contains.” However, Exchange will try to be helpful by replacing any alias SMTP addresses with the primary SMTP address. This makes the condition useless in our scenario. The way to work around this is to build the conditions based on the “When the message header contains” option.
Create a condition as follows: “When the message header (To) contains olddomain.com.”
This causes the transport rule to inspect the headers directly, which in turn will show the real address that the message was sent to.
New TransPort Rule - Specify Message header
On the following screen in the wizard, you will need to specify an action to perform on the messages that meet the conditions in your transport rule. I opt for the disclaimer text option, prepending the text to the message so the recipient doesn’t miss it. You can use basic HTML to reduce the font size and make it less intrusive.
New TransPort Rule - Append disclaimer
Once the transport rule has been saved and enabled, anyone that receives a message sent to your old domain name should see the warning text of your choice at the start of the message.
At this stage, it’s up to the recipient to ensure that those sending emails to them start using the new domain name before it is taken out of service.