Emails larger than 25MB may be lost or "blackholed" when forwarded to Salesforce. For example, oversize emails are lost when they follow this path:
Sender > Org Mail Server > Salesforce
but an NDR is returned when the same email is sent direct to Salesforce:
Sender > Salesforce
Your organization's mail server may not be sending an NDR due to backscatter prevention. For information on backscatter prevention, see http://en.wikipedia.org/wiki/Backscatter_(email)
Contact your organizations IT department or mail server administrator and ask them to set a limit of 25MB on email received to your public facing email address(es). This will ensure that oversize emails are stopped at your organization's mail server and an NDR is sent from your mail server rather than the emails being passed on to Salesforce.
The following tips are provided as is not supported under Email to Case Premium support.
In Microsoft Exchange you can set message size restrictions for individual mailboxes or contacts. In Microsoft Exchange 2010 server this is configured under the Contact or Mailbox properties > Mail Flow Settings tab > Message Size Restrictions.
The easiest way to address this in Gmail is by creating an attachment compliance filter. The filter will check all incoming mail to the email address you'd like to monitor, and is triggered when a certain criteria you designate is satisfied. Then, it automatically sends your customized response back to the sender. Messages that do not meet the criteria will complete their transit to destination.
Important: The following filter applies at the organization level. If you wish to apply this filter to a particular email address, create a new organization on your account and move the user to that organization. Then, with that organization selected, proceed to create the filter. Otherwise, the filter will monitor all incoming messages for the entire organization.
For more information about organizations please review the following Gmail Help Center articles:
Creating the filter
This filter will take a few moments to take effect. To test, use an external email address to send messages over 25MB in size, and verify the email is bounced back.
This behavior is, by design, due to backscatter prevention in many email servers.
For information on backscatter prevention, see http://en.wikipedia.org/wiki/Backscatter_(email)