First Method:
- Sign into the Office 365 Portal.
- Locate Users > Active Users or Groups > Shared Mailboxes if this is a shared mailbox.
- Select a user who has a Microsoft Exchange mailbox.
- On the menu on the right, locate Mail settings > Automatic Replies
Second Method:
- Sign into the Office 365 Admin portal using your admin credentials.
- Expand the Admin Centers panel, then select Exchange.
- Click the picture in the top right-hand corner. Select Another User, and then select the user mailbox that you wish to change.
- On the left side, select Options, click on Organize e-mail, and then click on Automatic Replies.