Emails from particular user are not getting tracked into CRM

Hello,

We noticed that emails from particular user are not getting tracked into CRM. All the other emails are tracked without any issues.

The below exception is logged in Event Log: 

Microsoft.Crm.Tools.Email.Providers.EmailException: The e-mail message with the details [Sender: mailbox@domain.com; Subject: FW: 111111111 Test Message Sent: 08/14/2015 04:03:21] could not be delivered because it does not contain a valid attachment. The message was moved to the Undeliverable folder. ---> Microsoft.Crm.CrmException: Invalid mail. Expected header items not found - possibly draft item or other type of item forwarded to this mailbox

Did anyone face this issue before ??