Reasons for Undelivered Mail

On occasion, mail sent to you or by you will not reach its intended destination.

N.B When mail is bounced, the bounce includes a diagnostic message that helps explain why it couldn't be delivered. That message is invaluable in diagnosing the problem.

We can, however, give you a general indication of the kinds of problems that prevent mail from reaching its destination.

Mail from you gets bounced
Mail to you gets bounced
Mail To Or From You Does Not Bounce, But Is Not Received
When in doubt, call us

1. Mail from you gets bounced

a) The Address is Misspelled, or the Address No Longer Exists
This can happen when you mistype an address or reply to a message where the sender's address has been misentered, and of course, it can happen when you're writing to someone whose address has changed.
If you're using a list or addressbook which hasn't been updated in a while, it's a good idea to check on them. It's always a good idea to update your address book or contact list when people notify you of address changes.
Errors of this sort will result in a diagnostic message that says "Unknown user" (or something similar), unless the address you're using actually exists on the receiving server. If the address does exist, your mail will reach whoever has that address, and you may or may not ever hear about it.
b) There's a problem with your recipient's domain
Mail delivery depends on the fact that the Internet has a system for figuring out where, on the network, a doman (the part of the address after the "@") is located. This involves two quite different details:
  1. The mail server that sends the mail has to be able to find the server for the domain that's supposed to receive it,
  2. The server has to be properly configured (with an "MX" record) to accept mail for that domain.
How can these go wrong?
For the first case, if the owners of the domain have not paid to keep it registered, the registrar for that domain will no longer provide other servers with the information ("DNS", or "domain name service") they need to deliver the mail. You can check on a domain's registration status with a whois search.
For the second case, even if the registrar is providing the necessary information, a server at the receiving end could be misconfigured in such a way that it doesn't accept mail for the addressee's domain.
c) The Recipient Server is Offline
The recipient's server may have become disconnected from the internet, either due to an internal fault or a problem with its external network connection. In this case, our server will make three more attempts to send your message over the next five days. Until and unless our server gives up, the messages you get will be a warning that the attempt has failed, rather than an outright delivery failure message.
d) Your IP Address has been Blocklisted
The IP address from which you connect to the internet can potentially end up on one of the blocklists. This can occur if a machine on your network has become compromised (with a malware infection that is automatically sending spam and/or copies of itself) or is being deliberately used to commit malicious acts against remote networks or devices.
e) Our IP Addresses have been Blocklisted
This is an exceedingly rare occurrence, and typically only happens if a computer on our network has become compromised and is sending out spam, or other abusive network traffic. We have monitors in place to alert us, we're normally aware of such an event long before it can adversely affect our users, and we take immediate steps to stop the offending activity (and prevent its recurrence), and to remove ourselves from the blocklist.
f) (AOL-Specific) "HVU:B2"
This is an error code specific to AOL, indicating that they have rejected your mail due to complaints they have received about a URL or domain in the body of your message. Unfortunately, they will not be specific about which URL, or why. If it's important that your correspondent at AOL receive the URL, you may need to find another way of transmitting it to them.
2. Mail to you gets bounced

You might not be aware that mail intended for you isn't reaching you. But sometimes someone will let you know that mail for you has bounced. If they can pass along a copy of the bounce message to you (or to Panix staff), it helps enormously.

But these are a few general causes:

b) The Sender's IP Address is Blocklisted
The same reasons why our IPs might get blocklisted apply in reverse. For a legitimately-run mailserver, this is (hopefully) rare and quickly remedied.
c) The Originating Server has Misconfigured Reverse DNS
Panix's mailserver is configured to reject mail from any server whose IP address cannot be reconciled with its hostname ("bad reverse DNS"). This problem has to be fixed by the server's own administrators.
d) Your Address has been entered incorrectly
Obviously, make sure that anyone from whom you want to receive mail has your address correctly recorded.
3. Mail To Or From You Does Not Bounce, But Is Not Received
a) Typos
Again, the wrong address will cause mail to get lost. In this case, the mistyped or mistaken address will be a perfectly legitimate one, and mail will not be "lost" so much as it is somewhere unintended.
b) The Mail is Filtered as Spam
Something about your message, or your correspondent's message, may be interpreted as spam by your respective filters. Assuming that the message is filtered into the Trash folder (rather than deleted immediately), it can be retrieved and moved into the inbox (or any other folder). We strongly recommend allowlisting the address of anyone whose mail you don't want to lose. Be aware that on Panix, mail sent to Trash will be automatically deleted after seven days.
c) The Recipient Server is Poorly-Run
A poorly-run server will sometimes silently fail to deliver mail. It may or may not be possible for the server's admins to diagnose why, given enough information about your email (i.e. the address from which you sent it, the exact date and time you sent it, etc). This assumes that the server is at least logging delivery attempts properly.

4. When in doubt, call us

If none of the above seems to fit, or you want to confirm your suspicions about a bounce, please call us at +1 (212) 741-4400), 10am-6pm (NYC time) M-F, or write ( We'll be better equipped to help you if you have a copy of the bounce message, but if the message reached our servers at all within the past three weeks, we'll be able to check our logs for clues about what happened.

Last Modified:Tuesday, 22-Feb-2022 08:47:30 EST
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