If you'd like to prevent Mimecast from re-writing the links in the Phishing tests you send, you can do so by adding KnowBe4's phish link domains as Permitted URLs in Mimecast. Our support team can provide you a list of our phish link domains. Simply submit a support ticket to www.knowbe4.com/support to request this.
Keep in mind, we don't recommend creating an exception for this unless you also have exceptions for other senders already in place. Otherwise, seeing anything other than a rewritten Mimecast URL will be a red flag for users and may skew your results.
Here is an article by Mimecast which will explain how to disable link rewriting on Permitted URLs: https://community.mimecast.com/docs/DOC-1029