Now it is safe to let customers start using the system again. Different services have different ways to permit this. However, it is often difficult to do testing without letting all users in.
There are some ways to do this, however. For example, when upgrading an email server, you can configure other email servers to not relay email to the server being upgraded. While those servers are holding email, you can manually test the upgraded server and then enable the surrounding servers one at a time, keeping a mindful eye on the newly upgraded server.