All the Office 365 email clients include an out-of-office assistant that responds to incoming messages with a note from you. When you use this feature, you have the option of enabling it at various levels.
Outlook on the Web allows you to work even when you're not connected to the internet. You can work with your calendar, read and reply to messages, and compose new messages while not online. The next time you go online, the work you've done with sync with your online account.
One of our recommendations for Calendar Harmony is that you send meeting invitation updates to all participants on all changes. This article describes how to have all meeting-related messages automatically filtered out of your Inbox and into a folder just for such messages.