Troops would often use commercial email accounts in order to avoid the filtering military intelligence does on all .mil email. This filtering also slowed down the delivery, when it was first introduced in the late 1990s. But mainly, the troops liked to conduct some business via commercial accounts to avoid any problems with the security people.
At many overseas bases, and some large navy ships, there are some computers available that are not connected to a military network, and the troops can use commercial email accounts on these.
Fearful of getting viruses, and other bad stuff, on military networks, the U.S. Navy and Air Force no longer allow their users to access webmail from government computers. The U.S. Army still allows this. This prohibition includes email websites of email providers like AOL, Google, MSN and Yahoo. The Department of Defense blocks a lot of web sites from their users, mainly the types known to secretly install ("drive-by" installs) software on the PCs of unwary visitors. This software is usually just to help send more online advertising to the user, but some of the drive-by software is crafted by criminals, or spies, to steal information from the users PC, and the network it is connected to. Military users can still communicate with family and friends, but they have to use their .mil email address. The military would risk a mutiny if they cut off email access for troops overseas, especially those in combat zones or at sea.