E-mail Rejection Explanation

As one should expect, our mail server won't accept e-mail that comes from invalid or non-existant sender addresses. Beyond the obvious tests that one can do (like making sure that the domain part of the address exists) a common technique is to perform a "callout" or "callback" test.

A callout test works by connecting to the mail server for the sender's domain name and going through the motions of sending a DSN (aka a "bounce") message to the sender's address without actually sending it. If the sender's address is invalid then the server will reject it, otherwise it should accept it.

If your e-mail is being rejected for this reason then you need to know that it's your mail server that is claiming that your address is invalid, not ours.

Now under certain circumstances you may find that your e-mail is blocked by this test even though your address is valid. This is rare, and when it happens it indicates a problem with your mail server. For example:

• Some mail servers will reject mail that comes from a "null sender", which is how the test is performed. The e-mail standards require mail servers to accept mail from null senders (RFC 1123 section 5.2.9) and failure to do so will result not only in failed callout tests, but it will prevent you from receiving error bounces, vacation messages, and various other legitimate mail.

• Some mail servers are implementing a new technology called SRS (Sender Rewriting Scheme) that involves replacing the sender's address with a different one. If this is done incorrectly then the address that they replace it with will fail the callout test.

If you are one of our customers and you are having trouble receiving mail from an ill-configured site like this, contact us and we will temporarily exclude the sender's mail server from this test.

If you are an end-user of a system with a broken configuration, please complain to your ISP or IT department and not to us. This is something they need to fix.

If you are a mail administrator and your server is rejecting messages from null senders you need to fix it. However, if you insist on operating a broken mail server, at least ensure that the rejection happens at the MAIL FROM: stage and not at the RCPT TO: stage where it is indistinguishable from a legitimate "no such user" error. And don't be surprised when more and more sites stop accepting your mail.

You can read more about our e-mail practices and policies here.