Email failure

After placing a message I get an error message about not being able to send an email...

A message that you sent contained one or more recipient addresses that were
incorrectly constructed:

New guestbook entry: missing or malformed local part (expected word or "<")

This address has been ignored. There were no other addresses in your
message, and so no attempt at delivery was possible.

What is wrong?

Regards, 10o.