This is a FASCINATING article, not only for the information on how Al-Qaeda communicated but how the information came into US hands.
A journalist, Alan Cullison, was replacing a broken computer in Kabul and purchased a stolen computer which had previously been used by top management of Al-Qaeda. It boggles the mind that such a random event provided the US with such a detailed view into that organization.
Inside Al-Qaeda’s Hard Drive [theatlantic.com]