This usually occurs when a user enters something other than a valid e-mail address in the "reply-to" field in their personal information.
You can find this configuration option by clicking on:
On this screen, you should see a text box called "Reply-To". This box should not contain your actual name, but, instead, it should either be left blank or filled with a valid e-mail address. Just leave this field blank in order for replies to be directed to your @purdue.edu e-mail address.