The Personal on Hold tab is where you will see all the emails that the filter is holding for 24 hours (those are the emails you see in the domain postmaster email you receive each morning). If you need to access an email that is on hold right away, click on the email you want and use the thumbs up button to either release or permit the email. 

If you are expecting an email and can't find it under Personal on Hold, that email has most likely been rejected or bounced by Mimecast because the system thinks it is a spoofing or phishing email. To check to see if this is the case, check under the Rejected Messages and Bounced Messages tabs. If your email has in fact been rejected, Micromenders can help release that sender for you so messages from them do not get rejected. 

If there is a specific email / sender you would like to block, simply email micromenders and they will block them for you.