Trainee invitations

when we shchedule an employee for a training the notification never reaches the trainee, instead the system administrators receive a message saying the original was a "Spoof". The full message is below. Anyone have a remedy for this?

"The format of the e-mail address is incorrect. Check the address, look up the recipient in the Address Book, or contact the recipient directly to find out the correct address.
< p01brzdns02.internal.breezecentral.com #5.1.3 SMTP; 553 The sender is spoofed.>"

TIA