No, this is not a “bombshell” email.  It’s connection to the possible security unpreparedness at the Benghazi Consulate is tenuous at best. An anonymous State Department official provided ABC News with an internal email where Undersecretary for Management Patrick Kennedy informed the security support team at the Tripoli Embassy that their request for continued use of a DC-3 to ferry them in-country was denied.
Since the plane in question is a transport plane and not a gunship, it would not have impacted the events of September 11 in Benghazi. Nonetheless, ABC News is reporting the email as evidence of something security-related:

ABC News has obtained an internal State Department email from May 3, 2012, indicating that the State Department denied a request from the security team at the Embassy of Libya to retain a DC-3 airplane in the country to better conduct their duties.

Copied on the email was U.S. Ambassador Chris Stevens, who was killed in a terrorist attack on the diplomatic post in Benghazi, Libya,  Sept. 11, 2012, along with three other Americans. That attack has prompted questions about whether the diplomatic personnel in that country were provided with adequate security support.

No one has yet to argue that the DC-3 would have  definitively  made a difference for the four Americans killed that night. The security team in question, after all, left Libya in August.

via Email Shows State Department Rejecting Request of Security Team at US Embassy in Libya – ABC News.
The fact that the security team departed Libya in August begs the question, if the security situation was deteriorating as some say, why was the security team pulled?
Jim Geraghty in his Morning Jolt column at National Review Online opined:
“But the question – both for the State Department, which is conducting an internal investigation, and the House Oversight and Government Reform Committee, which is holding hearings next week – is whether officials in Washington, D.C., specifically at the State Department, were as aware as they should have been about the deteriorating security situation in Libya, and whether officials were doing everything they could to protect Americans in that country.”
Maybe when congressional hearings begin next week, we’ll get answers, instead of leaks and political grandstanding. The families of Ambassador Stevens and his three colleagues deserve explanations.