Mike Krieger wrote that Army Secretary John McHugh certified to Congress that the Army’s approach to is in its best interests technically and financially.
McHugh also told Congress that the move provides the maximum amount of competition.
In February, Krieger announced the Army put the migration on hold so it could address requirements outlined in the fiscal 2012 National Defense Authorization Act.
More than 310,000 email users are on either the joint or Army enterprises, Krieger wrote Tuesday.
All migrations are scheduled to finish by the third quarter of fiscal 2012.