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
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
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.
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
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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