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To overcome the increased amount of spam received at out mail servers, we have introduced some additional rules for accepting or denying incoming mail.

Please have a look at the following page for a technical description of the current configuration: Anti-SPAM measures at Jacobs University Mail Servers

A side effect of this measure is that mail from certain remote senders is not accepted anymore. These senders receive a notification that their mail has not been delivered and why that happened.

An example is shown here:
 This is the Postfix program at host
 I'm sorry to have to inform you that your message could not
 be delivered to one or more recipients. It's attached below.
 For further assistance, please send mail to <postmaster>
 If you do so, please include this problem report. You can
 delete your own text from the attached returned message.
                       The Postfix program
 <>: host[]
     said: 554 Service unavailable; Client host [] blocked using; Spam Received withing last 2 days See: / Spam Received Recently
     See: (in reply to RCPT TO

The mail server - here - tried to deliver a mail to As the mail server was listed as spam source at that time, our mail server did not accept the mail and gave the cause, that the "client host" was blocked using this list, and that there is further information by following the supplied link.

Other causes of rejection



450 <>: Recipient address rejected: Domain not found

typo in mail address = the domain does not exist

450 <>: Sender address rejected: Domain not found

most probably a typo in the sender mail address, or spam

504 <Scanner>: Sender address rejected: need fully-qualified address

The mail client did not supply a fully-qualified domain name (FQDN) with the HELO command, as requested by the SMTP standard RFC2821

450 Client host rejected: cannot find your hostname,

There is no reverse name mapping for the IP address of the delivering mail server. This is not requiered by RFC2821, but it is best practice to deny mails from such sources. Most often these machines are pirated PCs from dial-up networks. If your party is sending from a "safe source", he/she should contact their local postmaster for help.

504 <vortex>: Helo command rejected: need fully-qualified hostname

The mail client did not send a FQDN with the helo command. If you are using MS Outlook and receive this answer after sending a mail, please configure your Outlook for authentication during sending mails. There is a HowTo, steps 11 and 12 are the most important ones.

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