This message indicates the operator has an invalid e-mail address, this is appears to be a response from your SMTP relay. Check the database mail logs. Try to reproduce with a new operator - i.e.
EXEC msdb.dbo.sp_add_operator @name=N'TEST',
@enabled=1,
@pager_days=0,
@email_address=N'someemailaddress@yourcompany.com'
Confirm you can access the SMTP relay from both nodes on the cluster -
http://support.microsoft.com/kb/153119
Good luck 🙂