Mimecast may 'sanitize' erroneous characters that are crucial to the appearance of HTML based email, resulting in odd characters in the email's body copy. Follow this guide to bypass this policy.
If your emails contain odd characters in the body copy, including this question mark symbol (below) follow these steps:
Log in to your Mimecast administration console.
Select Administration, then select Gateway, then Policies.
Scroll down and select Message Sanitization.
Options
Click New Policy.
In the Policy Narrative field, type Nectar IP Permit. (Or whatever will help track this policy.)
In the Select Option field, select Take No Action.
Emails From
In the Addresses Based On field, select The Return Address.
In the Applies From field, select External Addresses.
Emails To
In the Applies To field, select Internal Addresses.
Validity
In the Enable/Disable field, select Enable.
In the Set Policy as Perpetual field, ensure the policy is set to Always On.
In the Policy Override field, check the box.
This overrides the default order that policies are applied. If there are multiple applicable policies, this policy is applied first unless more specific policies of the same type are configured with an override.
In the Source IP Ranges, input Nectar's delivery server IP address:
159.183.82.50
Select Save and Exit
Now you've configured Mimecast to allow emails sent using Nectar to bypass Message Sanitization!