The purpose of mime cast is to block or permit email messages to come into your email inbox.
First step is open up Outlook. In the top menu click on Mimecast.
Click on Managed Senders
If you want to permit emails, click on Permitted, click on +Add Permitted, enter the email address or domain that you want to add to the permitted list. If you want to add a domain just enter the domain such as gmail.com or aol.com and this will permit all emails coming from gmail email address or aol email address.
If you want to block an email or group of emails, click on blocked, Click on +Add Blocked, add the email address you want to block, click ok, or if you want to block a group of emails enter the domain name such as gmail.com, or aol.com, or att.net, ect.... (these are just examples)
If you are getting an authentication error with mimecast, click on Mimecast, click on account settings, attempt to login.... if this works under Cloud Authentification it should say "Active". If you cannot login let your local IT person know and they will need to uninstall Mimecast and re-install it