The Private Message Center (PMC) is a part of OPTIMAL that you can use to securely communicate with the people you've given a Guest Account to. When you give someone a Guest Account they'll be able to use the message center to leave you a quick note, make arrangements with you, or collaborate with you on schedule changes.
PMC: What It's Not
It's not email. Your messages stay on our site, so they're generally more secure than email. If you want, you can choose to have notification of new messages sent to you by configuring the PMC options.