Before taking any action that can cause a long-term interruption in service, attempt to inform the customers that there is a problem being worked on. Notification consists of an update to the daily log system and any other status applications in place.
If the affected server is in a state that messages can be sent to the mailboxes, use an announcements-type mailbox or the server's test mailbox to send the message. Essentially, you want a mailbox named something like "Messaging operations." This gives users a good reason to read the message.