unable to relay mail

Power failure. Servers down. Bring all servers up. ISA in front of Exchange 5.5 SP4. Mail not being received

“””Message from yahoo.com.
Unable to deliver message to the following address(es).

:
69.7.200.66 does not like recipient.
Remote host said: 550 5.7.1 Unable to relay for
lheisey@microplasticsinc.com (this is me of course)
Giving up on 69.7.200.66.””

So what is up with this? What does it mean? None of the
mail from Friday made it to us. Never backed up on the ISP’s server. They said once connection was made here it bounced back to SENDER. Is that theory correct?