E-Mail typically used at work to get out of work or justify absences.
Typical Flee-Mail sent at work: "I have a (Doctor, Dentist, etc) appointment", "I'll be out of the office on (Vac, Personal day, Sick Leave, etc", "I'm taking a Late Lunch.." "I'll be leaving the office due to a personal emergency, etc..."
An escape from a convoluted, multi-addressed chain of email that no longer interests or involves the writer in any meaningful way.
"Thanks, all of you, for your many and varied contributions, but I note that we have reached a level of detail on insect migrations that I no longer find compelling or meaningful. Please consider this my fleemail and eliminate me from further cc lists, but do continue blathering amongst yourselves."