SMTP error

hi all,

We provide wireless internet service in a % star hotel.We have our own smtp mail server for guests wanting to send their mails out thru different accounts.We just change the users smtp add in OE6 and outlook,eudora etc.mail clients.
Recently i am observing that many of the mails dont pass thru the smtp server and the users get an error 553 ” soory this domain in not in my list of allowed RCPT hosts “.
The strange thing is that for some intervals during the day the emails pass thru with the same settings.