There are a number of reasons as to why emails may not by received by the end recipient:
1. It may not have been sent from Outlook correctly.
2. It may have been rejected by the recipients mail server because it contains an encrypted .switch attachment. The server does not recognise this and therefore sends it to quarantine.
3. It may be in the spam or junk folder of the recipient.
4. It may be held up by the mail server of the sender and has refused to deliver it to the inbox.
If the issue is related to the mail server, their internal IT department will need to unblock, and/or whitelist the emails to then reach into the recipients inbox.