If you are sending emails from Dynamics CRM, there is the possibility that the process of sending the mails fails. This article should give you a short overview on existing articles, which are describing the debugging details.
In order to look whether there are pending or failed mails in your system, you can use following search. As you can see, it is based on the existing view ‘My pending E-mails’. You simply have to remove the ‘Owner = current user’ condition.
If the result of this search contains mails which are in pending state since ages or have failed to send, it’s debugging time again. If you are using the Outlook Client for sending the mail, then take a look at http://blogs.msdn.com/emeadcrmsupport/archive/2009/12/18/how-to-activate-crm-4-0-platform-tracing-on-server-client-and-e-mail-router.aspx for a description on enabling the tracing.
The debugging on the E-Mail Router side is a little bit more complex, but it is explained in detail in this article http://blogs.msdn.com/benlec/archive/2008/03/25/how-to-implement-logging-for-the-microsoft-crm-4-0-e-mail-connector.aspx