The email was first highlighted in the highly critical State Department Office of Inspector General report. Just after it was released Justin Fishel at ABC News pressed the State Department at this press briefing about why it was not among the Clinton emails previously disclosed and an unnamed State Department official said they weren't sure where it came from, but that the emails previously released were only the ones Clinton had handed over.
The production of the full email appears to answer the mystery - it is marked in the lower right with "HA 09/01/15" which seems to indicate it was part of the September 1, 2015 production from Huma Abedin described here (p. 4) that included "1.4 gigabytes of electronic files containing 348 pages of documents and 6,714 emails." The State Department confirmed this to Tau.
This email in particular is important because - while it is not a slam dunk by any stretch - it is evidence of Clinton being motivated by something more than mere "convenience" in her choice to use a personal email address for State Department business. The fact that Clinton did not produce it raises legitimate questions. It was not from the time prior to March 2009 when some Clinton emails were missing as a group, nor is it from any of the periods of time in which the National Archives identified "gaps" in the Clinton production.
The remaining possibilities are either that (1) Clinton deleted it early on in the normal course of business or (2) that Clinton and her attorneys categorized this email as "personal" and then destroyed it.
(1) Clinton deletion in the ordinary course of business
This possibility seems less likely based simply on the fact that in reviewing Clinton's other emails, she seems to be a bit of an email hoarder. Consider, for example, other Clinton emails from the same day - they include a one liner from Jake Sullivan about "Denis" still being in a meeting, an email from Clinton about whether she should accept an award from designer Diane von Furstenberg, and another email from Huma simply forwarding a news article about the French government. It is odd that Clinton retained this last email with the generic news article about France, but did not retain any part of the email chain that is the subject of this post which begins with scheduling a call with the resigning French Minister of Foreign Affairs Bernard Kouchner, which is arguably more substantive. Other email chains from the same day are also produced as a series of individual emails, which highlights the fact that if Clinton decided to delete this email in the normal course as an unimportant scheduling email she apparently would have been deleting four emails - two emails from her inbox and two emails from her outbox. It remains a possibility however.
(2) Destroyed as a "personal" email
The other possibility is that this email survived and was reviewed by Clinton's attorneys. Under the detailed description Clinton has provided about that process, this email should have been captured. According to Clinton, her attorneys identified any potential "non-'.gov' correspondence" by searching for first and last names of State officials including all of Clinton's close aides and staff, which should have included Huma and should have included the emails in this chain. Clinton's current description also expressly states that "each email" was reviewed "erring on the side of including anything that might be even potentially work-related." Again that would pretty clearly seem to include the emails in this chain.
Yet the detailed review of "each email" is actually what concerns me here. I presume that these emails were reviewed by Clinton's lawyers for production in a manner that is very similar to the production of documents in litigation. It would not surprise me if there were rough categorizations into a few different piles, one of them being a "hot documents" file for close review. And it is not difficult to imagine Kendall and/or Mills and any junior attorney working on these emails having a meeting where they specifically discuss this email, that it is an inconvenient document that could be misinterpreted, that it is an email between two clintonemail.com addresses, that on its face it is not clear what "Kouchner" is being referred to here, and it is explicitly referring to "personal" information and after long analysis and thought they "get comfortable" that this goes in the "personal" pile for eventual destruction that would preclude second guessing.
If this sounds far-fetched, I would simply note that Kendall's letter (last attachment in this filing) describing his interpretation of the federal records laws can charitably be described as extremely aggressive (although I would go as far as to say manipulative and clearly wrong). He was of course simply doing his job as an advocate for his client, but this is precisely why he should not have been making records determinations on her behalf and why Clinton has exposed herself to allegations, whether real or conspiratorial, that her emails were selectively sanitized and federal records were destroyed.