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a2i communications explanation of mail errors

The mail software at a2i may reject incoming mail if it cannot verify the validity of the sending address or domain. Here are explanations of some of the error messages. Also, for current information about our anti-junk-mail blocks, see our Nojunk(tm) page. If you are trying to send mail to our machines from a spam-friendly service such as CompuServe or Netcom, or from any service that any specific time is engaged in mailbombing our machines or sending us a high volume of junk email, you may encounter some difficulty getting through.
rejected: cannot route to sender
The sender address is invalid for some reason. The message has been rejected. This error usually occurs when the sender address contains an invalid (nonexistent) domain. The machine that tried to deliver mail to a2i will try to bounce the message back to the sender. To solve this problem the sender should contact his ISP and ask for a fix, such that outgoing mail will show a valid sender address.

NOTE: This error can also occur if a sender uses an "antispam" email address such as josuser@REMOVETHIS.example.com. If an attempt to do a name server query for the domain REMOVETHIS.example.com results in a name server error, the message will be rejected. To avoid this sort of problem, any antispam fragment should be on the left side of the @ sign. e,g., joeuserREMOVETHIS@example.com. Better still, such antispam fragments should not be used in email at all, since they prevent the recipient from replying, and provide little or no protection against junk email. (Junk emailers harvest addresses from Usenet, not from private email.)

temporarily unable to verify sender address (try again later)
This is similar to the 'cannot route to sender' error, except that the error is temporary. The sender's domain appears to exist but its name servers are not currently responding. The sending site will normally keep the mail queued and it should eventually get through. If it does not, the sender should contact his ISP and ask for a fix (or correct any antispam address as described above).
relaying to xxxx denied, or refused relay
The a2i host refused to relay mail for a third party. Mail is accepted by a2i machines either if the sender is in a local domain, or if the recipient is in a local domain. If both sender and recipient are in nonlocal domains, the software assumes that the sender is a spammer trying to relay junk email through the a2i machines to random addressees elsewhere on the Internet. If the sender is in fact an a2i customer, he should make sure he has correctly configured his software as described in our a2i Servers web page.

We have also had reports that some sites are using software with bugs that takes an address ending in xx.US and appends .COM to it, generating an incorrect address of the form xx.US.COM. The buggy software will then try to deliver it to our machines, which are MX hosts for *.US.COM, and will get an error message denying relaying. The sending site should correct its software.

rejected: administrative prohibition
The incoming message was rejected by our SMTP server for one of the following reasons: (a) it came from a network that has been blocked at a2i, usually due to spam; (b) it came from a domain that has been blocked at a2i, usually due to spam; (c) it came from a sender who has been blocked at a2i, usually due to spam. If the sender is not a spammer and urgently needs to get in touch with the recipient, he should either make a telephone call, or get an email account at one of the various free email providers (such as www.hotmail.com or www.juno.com) and send a message from there. A blocked sending organization that believes that it did not originate or relay spam should report the problem to a2i (see contact information).

Except for selected permanent blocks, most SMTP blocks are automatically removed after three days, or sooner if a customer so requests. However, if the same source of junk email remains active, the block will normally be added back. For a permanent solution senders at blocked sites should get in touch with their service provider and ask it to take steps to not let its hosts send junk email to our machines.

Here is a log of some recent SMTP rejections.

rejected: no valid header sender, or can't verify header sender
The incoming message was rejected because no valid reply address was found in it. Every mail message should have a valid From: line with an address that appears to be syntactically correct and with a valid domain name. If the address in the From: line is not valid, then a valid Reply-To: address will be accepted by our software. If a person sending mail uses an antispam address, it must be properly formatted to use a valid domain, as described above.
554 SMTP service not available for host [x.x.x.x]
In most cases this error will be generated if the IP address x.x.x.x could not be mapped into a host name. Our mail hosts will not accept SMTP connections from a host whose name cannot be identified. This usually happens if reverse DNS is not correctly working for the host that is trying to send mail to our machines. The sending site should check and correct its DNS configuration, possibly with the help of its Internet service provider. If this is not possible, the sending site should arrange to relay its outgoing mail though some other machine that does have correctly working DNS.

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