When a classmate tries to log on she gets this message...You are logged on as a different classmate than the recepient of this message...What to do...

Tell her/him to log out and then click the e-mail again. Probably what is happening is that they married a classmate. The spouse/classmate forgot to log out. So when the classmate tries to read the e-mail, they aren't the person that the e-mail is for, so they cannot read it.