The defaults on the Out of Office reply are to not reply while someone is actively logged in to the mailbox. Once the User logs out, the Rule becomes active. You can change this behaviour if you want by editing the Rule directly -- right-click on the mailbox and select "Edit Rules". From here you can edit the Out of Office Rule and uncheck the "while there are no active logins" box if you wish.
Also, the Out of Office Rule will never reply to yourself. So you can't send yourself an e-mail message from the same mailbox and expect a reply to that same mailbox -- those kind of looping replies are considered bad form.
If you want to test someone else's Out of Office reply, just send them an e-mail message from your mailbox (as long as you don't have their mailbox in your E-mail panel). To test your own Out of Office reply, you would need to send an e-mail to yourself from a different e-mail address.
Also, the Out of Office Rule will never reply to yourself. So you can't send yourself an e-mail message from the same mailbox and expect a reply to that same mailbox -- those kind of looping replies are considered bad form.
If you want to test someone else's Out of Office reply, just send them an e-mail message from your mailbox (as long as you don't have their mailbox in your E-mail panel). To test your own Out of Office reply, you would need to send an e-mail to yourself from a different e-mail address.