Summary:

A user could not file and send a new email to CRM in outlook.
But a user could file a received email to CRM so this suggests that the issue is with the outlook settings and not CRM

Workaround / Resolution:

The issue here happened when two mail boxes were set up in Outlook.
CRM is not supported to use multiple mailboxes.

The issue can be resolved in Outlook by going into "File" and "Account settings".
In the account settings set the email address you are using as the default under the "E-mail" and "Data Files" tabs.

In this case the user had set the email address as default under the "E-mail" tab but had a different one set as default under the "Data Files" tab. This is why the user could not send and file a new email to CRM.