Both Mailguard and Mimecast have a list of allowed emails for users. When migrating from one platform to another you will need to copy these over.
Mailguard does not have an export function for its “Active Whitelist” so you will need to copy the Table produced in the Admin Panel into Excel and remove all but your two columns of emails.
These two columns will need some more manipulation as they mix up Senders and Receivers in the lists and Mimecast needs one Column for Each. But the First Column in A in Excel and he Second in B
In C1 add the following ( If A1 does has @domain.com in it list it , if B1 has @domain.com in it list it )
In D1 add the following ( If A1 does not have @domain.com in it list B1 , if B1 has @domain.com in it list A1)
Once one , create a .xls file with columns
Add the domain.com to #Address
Add the other domain field to either trusted_senders ( Allow Spam and Attachments ) or Approved Senders ( Allow Spam )
Import into Mangaed Senders using the PostIni Option