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Inbound mail from one sender seems to be vanishing. What is happening?

Are you in a situation where mail from a specific sender never reaches SpamHero and the sender doesn't receive a bounce message? There's a very common cause for this issue.

First, check the quarantine

If the message doesn't appear in the SpamHero quarantine (neither the "clean mail" nor "spam" view) then it means the message didn't reach our servers. If the sender doesn't receive a "bounce" or "failed delivery" message then it usually means that the message was delivered to the wrong server.

In some cases, this scenario only happens to messages that are sent from web-based apps (such as webmail) or contact forms.

The problem: email misdelivery

In 9 out of 10 cases, this scenario indicates that the sender's mail server is delivering the "lost" messages to a local mailbox. This commonly occurs when the sender and recipient are both using the same "shared" hosting provider.

Why does this happen?

When a mail server receives an "outbound" message it checks whether the recipient's domain is handled locally. When a recipient domain is considered "local" the MX record lookup is skipped and the message is delivered to a local mailbox.

Even if the recipient isn't using the same mail provider, it can still have the same cause

There are a couple of cases where this still applies:

  1. The recipient's website is hosted by the same provider even though their mail is hosted elsewhere (e.g. Microsoft 365 or Google Workspace). The MX records may point to their "real" mail server but the hosting provider is still configured to handle the mail locally.
  2. The mail provider previously handled the recipient's mail and failed to remove them as a "local domain" when their hosting service was canceled.

Another fairly common (but unrelated) issue is that the mail provider simply has a misconfiguration that is causing mail to be routed to the wrong host. Or, in some cases, they have a spam filter (or even an advanced firewall) that is intercepting the messages.

What is the fix?

The solution depends on your particular circumstances. The most common solution is for the sender's hosting provider to disable local mail delivery for the recipient domain (in cases where they are no longer the email provider for the recipient domain) to ensure that mail is relayed to the designated MX records.

Ultimately, if the target message is not showing up in the quarantine (neither the "clean" nor "spam" view) then it means the message is not reaching our servers and we do not have any way to track it down. In other words, the sender will need to work with their outbound mail provider to solve this.

Additional troubleshooting

Only the sender's mail server can definitively identify what happened to the missing message. If the sender's mail host claims that the message was "successfully delivered", request that they provide the associated mail logs for further analysis.

If you would like help identifying the path of the sender's outbound mail, contact us to get a special testmail@ address that they can CC for additional insights.

Last updated October 25, 2023