We’re currently going through a migration from our existing legacy email provider to using Mimecast as our SPAM filter. We have some services which we can’t interrupt without planning, so need to deploy Mimecast to Office 365 for our ‘user’ domain without disrupting our ‘alerting’ domain. We also need to validate Mimecast configs and setup before impacting on users, so we also have a test domain to verify configuration.
We therefore wanted to add the email filtering staged in the following order over a number of days:
However, the Mimecast documentation isn’t great for describing split routing of email based upon the senders domain, and essentially assumes that you want to send all email out through Mimecast from the off.
This great article from Antonio Vargas really helped us out in understanding why the rule wasn’t intercepting messages from the domains to send out.
In the Conditions select “Apply this rule if..” > The recipient is located > Outside of the Organization
Once that was applied to our rule, we were immediately able to verify that the test domain was able to then route email through Mimecast from Office 365.