iCloud: Set up an automatic reply in Mail on iCloud.com
You can reply to emails automatically. You can use this feature even if you forward your iCloud Mail to another email account.
Send an automatic reply
In Mail on iCloud.com, click the Action pop-up menu in the sidebar, then choose Preferences.
If you don’t see the sidebar, see Show or hide the sidebar in Mail on iCloud.com.
Click Vacation, select “Automatically reply to messages when they are received,” then type the message you want to send.
Set an optional date range; the auto-reply will be active from the first day of the date range until the last.
Anyone who sends you multiple messages while you’re away will receive one auto-reply in a 24-hour period.
Stop sending an automatic reply
If you don’t set up a date range when you set up your auto-reply, you can turn it off manually.
Important: If your mailbox gets full, you won’t receive new messages. If you’re going to be away from your email for a while, make sure you have space available for new messages.