I can think of a bit of a hacky way to do this, but it might leave you with undesired results (read: run this command at your own peril). You don't specify the size of your organization, but this is probably only really appropriate for a small number of mailboxes.
You can manipulate the mailbox auto reply configuration (out of office) using PowerShell, which means you are able to change the OOF message and turn it on in bulk.
I'll put the warnings about running the command before actually posting the command, just so you read them.
- This will change the users OOF message - it is not a temporary message
- If the user has rules configured to forward messages to another user when they are OOF they will become active. I haven't tested it and don't know how Exchange will react to a forwarding rule such as
User 1 <==> User 2 (User 1 forwards to User 2, and User 2 forwards to User 1)
- If you have a user who is out of the office for an extended period (maternity leave for example) the Christmas OOF message will continue to be sent until it is turned off.
In the Exchange Management Shell, run
Get-Mailbox -OrganizationalUnit "Acme Widgets Users" | Set-MailboxAutoReplyConfiguration -AutoReplyState Enabled -ExternalMessage "The office is now closed until <<some time>>. Your email will be dealt with on my return."
Change the message as desired, and if you're feeling particularly adventurous you can change the condition (the preceding
Get-Mailbox) to something more suitable. Please also test the command without the
Set-MailboxAutoReplyConfiguration to see what users will be affected.