Miscellaneous
	Undelivered e-mails

Undelivered e-mails to our department.

Was your e-mail returned to you or was your e-mail lost or not delivered to someone in our department? Here are the possible reasons. We order the explanations according to their likelihood. The most probable reason is mentioned first.

• The sender is using on his or her own side an e-mail server which does not have a reverse DNS. In this case the sender immediately receives the following error message

  550 Client host rejected: cannot find your hostname, [xxx.xxx.xxx.xxx]
  

  If the sender does not clearly understand the message the sender should contact his or her own system administrator. The administrator will fix the problem, namely he or she will register the SMTP client (from where we receive the e-mails) to DNS. The administrator should register not only the direct but also the reverse DNS. Please note if we are unable to resolve the domain name of the SMTP client based on its IP address we will not communicate.

  We would like to stress the fact that the error is not on our side, but it lies in the configuration of computers from which the e-mail was sent.

• The sender is using an e-mail server on his or her own side which is blacklisted in spam listings. In that case the sender immediately receives the following error message

  
  554 Service unavailable; Client host [xxx.xxx.xxx.xxx] 
  blocked using dnsbl.sorbs.net;
  See: http://www.dnsbl.sorbs.net/lookup.shtml 
  

  Besides dnsbl.sorbs.net the message can indicate list.dsbl.org which is another data base of blacklisted listings. The sender should contact his or her own administrator to remove the SMTP client from the blacklist.

  Please note the error is not on our side, but it lies in the configuration of computers from which the e-mail was sent.

• The sender is using an e-mail server which is blacklisted in the relay networks. In that case the sender immediately receives the following error message

  554 Service unavailable; Client host [xxx.xxx.xxx.xxx] blocked using 
  relays.ordb.org; This mail was handled by an open relay - 
  please visit http://ORDB.org/lookup
  

  The sender should contact his or her own administrator to remove the relay loop in the network, and then remove the SMTP client form the blacklist.

  Once again, please note, the error is not on our side, but it lies in the configuration of computers from which the e-mail is sent.

• The sender sent an e-mail which in many aspects resembles a SPAM e-mail, or sent an email which contains a virus in the attachment. In this case the sender does not receive an error message. The e-mail is delivered to our 'e-mail quarantine' and not to the intended user. This 'e-mail quarantine' is erased on weekely basis, of course we only erase e-mails week old or more. If questions persist selected users on our side can view the 'e-mail quarantine'.

  However, no 'useful' e-mail was lost due to this reason yet. This reason is highly unlikely. The above mentioned items are far more likely.

Note: It can happen the sender fails to receive error messages we mentioned. Either he or she receives a 'changed' message or no message at all. This is attributed to an error in the computer configuration on the side of the sender. We close the communication with the SMTP client only after we send a correct error message to the client. Please note we can not be held responsible for the fact the client hides the error message from the user.

System administrator: Petr Olšák