In addition to the recent updating of the Document Image System (DIS) Implementation Guide, US Customs is streamlining the DIS email process.
Per CSMS #16-000370:
The [DIS] is transitioning to only one email box (email@example.com) and one format for most email submissions to DIS. Please review the DIS Implementation Guide on cbp.gov for proper email box, format, document codes, and document labels. Beginning May 28, 2016, Trade participants will no longer be allowed to submit emails to the following emails boxes: CEE-DOCUMENT-SUBMISSION@cbp.dhs.gov and SimplfiedEntryDocs@cbp.dhs.gov.
Additional information available here.