Login to the shared mailbox you have access to via the web by going to http://outlook.rowan.edu/resourcename@rowan.edu.
So, for example, http://outlook.rowan.edu/admissions@rowan.edu for the Admissions Resource mailbox
Login with your own username and password.
Click the wheel icon in the upper right next to the name of the mailbox and then select "Set automatic replies"
On the "Automatic replies" screen, turn them on by clicking the "Send automatic replies" button. Choose a start and end date if applicable.
Enter your message and then choose whether to send replies to all external users or just external users in your Contacts. Rowan users will receive the automatic replies by default.
Once finished, click "Save."