exit memo
n. A memo addressed to the employees of a company, written by a person about to leave the company.
Among the memoranda and spread sheets that make up most corporate communications, the goodbye memo — mass-mailed to colleagues on an employee's last day — has become a kind of mini-genre of its own.

Against the clear advice of career counselors to hold one's tongue and not send anything but contact information, many people are unable to resist waxing sentimental, self-indulgent or bitter, especially in the case of a layoff. This holiday season, typically a time of departures as executives collect year-end bonuses and move on, the sputtering economy seems to have created a thicker-than-usual stream of exit memos.
—Katherine Rosman, “They Got Mail: Not-So-Fond Farewells,” The New York Times, December 01, 2002
1990 (earliest)
Ironically, one of the biggest problems cited by midlevel associates at Willkie Farr & Gallagher is senior associates who seem "covetous of their relationships with clients" to one midlevel, and "worried about their futures" to another.

But the midlevels are worried, too. Willkie has slid steadily down on our charts since 1986, when it was ranked number six in the city. This year its rating of 3.12 puts it in thirty-third place citywide and fourth from the cellar nationwide. One respondent raved that the "complexity of the work is wonderful, and the learning curve is tremendous in the early years. The pay is fantastic. However, you never get over the feeling that you are one in a long line of bodies that have filled your office, a feeling that is made stronger by the never-ending stream of 'exit memos' in your in box." In fact, the firm scored a low 2.6 for the likelihood that associates will remain for two more years.
—“Midlevel associates 1990,” The American Lawyer, October 01, 1990
Synonyms for the exit memo are the goodbye memo (1988) and the departure memo (1989). Nowadays, exit memos are most often sent via e-mail, so some variations on the theme are exit e-mail (2002) and goodbye e-mail (1992).