NARA Preferred and Acceptable Formats
National Archives and Records Administration has a widely referenced Appendix to its Bulletin 2014-04 on Revised Format Guidance for the Transfer of Permanent Electronic Records. Appendix A, which is continually updated, lists both preferred and acceptable formats for transferring electronic records. Formats are listed in 10 categories:
4. Images / Photos
5. Geospatial Formats
6. Presentation Formats
7. Textual Data
8. Structured Data Formats
10. Web Records
In addition to preferred and acceptable formats, the Appendix also sometimes references acceptable imminent transfer formats.
So for example for email the preferred formats are .eml and .mbox, and acceptable formats are .xml and .msg formats. NARA requires that email include delimiters that indicate the beginning and ending of messages and attachments, and "Labels to identify each part of the message (Date, To [all recipients, including cc: and bc: copies], From, Subject, Body, and Attachment) including transmission and receipt information (Time Sent, Time Opened, Message Size, File Name, and similar information, if available)."