If mail is being regarded as spam and it shouldn't the user can configure this in Outlook. However if a lot of users are complaining about this, it may be easier to create a transport rule in Exchange.
Open the Exchange Management Console and navigate to Organization Configuration, Hub Transport, tab Transport Rules. From the Actions pane on the right, choose "New Transport Rule":
Enter a description for the new rule and make sure it is easily understandable! Press next to continue.
Select the "when the From address contains specific words" option and in the Step 2 windows add the domain name to be allowed (in this case thisisnotspam.com ). Press next again.
In this screen select the option "Set the spam confidence level to" and in the step 2 screen make sure it is set to 0 meaning it isn't spam. Continue pressing next a couple of time and then finish the wizard. Once the wizard is closed, the rule is active and the given domain won't be regarded as spam anymore.