Because you'll want to see for yourself, your first action upon receiving this message will probably be to check your use of resources by navigating to 'resources in use' (in CRM 2013 the path is
settings => administration => resources in use. In CRM 2011 it's settings => system => administration => resources in use)
You'll see something like this:
Email attachments are often the culprit. To delete an email attachment, but not the original email, it's a little tricky. First, download and install this handy plug-in:
Then create an on-demand workflow. In your Workflow Steps, you'll see a new menu item at the very bottom (make sure your task bar isn't preventing you from seeing it!) called Lat Workflow Utilities. This was put there by the plug-in you just installed. Drill down and select Delete Email Attachments:
Next, specify the properties:
I used an attachment size of 2000 bytes here. Now save, activate and publish your workflow.
Next, find your emails with the large attachments using Advanced Find. In the screenshot below you'll see that I want a list of all emails with attachments larger that 1,000,000 bytes:
Select one or more emails from the list (my list below has only one email). Click on the Run Workflow button.
You'll see the workflow you created earlier in the list:
Click Add and your email attachment(s) will be removed. Note that the email attachment is removed from CRM only, not SharePoint or Outlook.