To update Mimecast password in Outlook

Click on Mimecast > Account Settings (or the pop up banner at the bottom right of outlook)

Click on Domain Authentication

Enter your own Citrix password and click Submit Password 

You will then be required to put in a verification code to authenticate your access which will automatically be sent to you from postmaster@guhg.co.uk

click Verify once you've entered your verification code

Mimecast setting should appear back now