Although there seems to be multiple causes of this error, one being security settings, this particular error applies when the following holds true:
1) You are receiving a failure message when trying to send e-mails with the 'track in crm' option marked.
2) You can view Microsoft CRM items while in Outlook (i.e. you can click on the accounts folder and see the list of accounts)
3) You can track other types of items in CRM, such as appointments.
4) You have an image imbedded in your signature on e-mails

Resolution:
Remove the image from your e-mail signature and test sending.  If this works then you need to compress the image further by changing the format and/or size.