When you send e-mail to sites which apply the S25R anti-spam system, your e-mail may be temporarily blocked with one of the following messages:

• 450 4.7.1 <unknown[***.***.***.***]>: Client host rejected: reverse lookup failure, be patient
  (Reverse lookup of the IP address of the sender host failed, or the retrieved reverse name doesn't correspond to the IP address. Please be patient for a while.)
  
  
• 450 4.7.1 <***-***-***-***.example.net[***.***.***.***]>: Client host rejected: S25R check, be patient
  (The reverse name of the sender host was trapped by the S25R condition. Please be patient for a while.)
  
  
• 450 4.7.1 <***.***.example.net[***.***.***.***]>: Client host rejected: domain check, be patient
  (The reverse name of the sender host was trapped by a domain condition. Please be patient for a while.)
  

"450" is a response code meaning "try again later". In this case, your e-mail is reserved in the sending mail server, and the sending mail server will try to resend your e-mail in a little while.

When you received a notice message in several hours (about 4 hours): It must not be a delivery failure notification but just be a message warning that your e-mail has not been delivered in several hours. Please be patient for a while without resending your e-mail. Your e-mail will be delivered when the administrator of the receiving mail server registers a permission condition for the sending mail server after finding retry accesses.

When you received a delivery failure message in several days (5 days in many cases): The administrator of the receiving mail server has not registered the permission condition for the sending mail server. It is mistaken operation of the S25R anti-spam system. Please notify me.