hide real e-mail address

Not sure where to post this question.
WLM 14v.
My alumni group website received an anonymous letter with invalid (fake ) address.
This alumni group site is Membership Only. Members must provide valid e-mail addresses and nobody is to send messages to the group without membership and valid acknowledged address.
So, my question is, how did this happen ? How did the message get through, and how did the sender use faked address and still be able to send the message out ? We want to stop this. Please advise. Thank you.
P.S. The anonymous message is NOT malicious. It contains concern of group's policy and requests changes. It is obviously sent by a current member. But still ........ did we get hacked ?