Emails Not being sent

When sending an Sales quote or Purchase order in Myob we send through outlook.

But some emails just disappear they are not sent to Outlook outbox. The email addresses are stored in each customer card and are legitimate email addresses, If we put our email address in it goes through fine and then we can forward using the customers address fine, but for some reason it will not send to their email address.

This happens to about 4 out of 10 emails.

Any help would be appreciated