Why was the message rejected with the error “550 5.1.1 User unknown in virtual mailbox table”?
This error means that the address used as the technical sender (Return-Path) does not actually exist on the domain’s mail server.
It commonly occurs when the message is sent from an automated sending platform (e.g., newsletter, online form, marketing campaign) that uses technical addresses for bounce management.
What is the difference between “From” and “Return-Path”?
-
From → The address displayed to the end user
(e.g., name@example.com) -
Return-Path → The technical address used during SMTP delivery for bounce handling and automated verifications
(e.g., bounces@example.com)
Security, anti-spam, and verification systems (such as CAPTCHA or SPF) often rely on the Return-Path because it identifies the actual sending server.
Why are CAPTCHA or SPF checks performed on this address?
During SMTP transmission, checks are performed on the envelope sender address (Return-Path), which represents the real sender of the message.
This allows mail systems to verify the technical consistency of the email (SPF, DKIM, DMARC, etc.), independently from the address displayed in the From field.
Can other similar errors occur?
Yes, other errors may occur depending on the issue encountered.
For example:
<no-reply@example.com>: Host or domain name not found.
Name service error for name=uhmailsrvc9.com type=A:
Host found but no data record of requested type
This error indicates a DNS problem:
- The domain exists, but it does not have a valid A or MX record.
- The destination server is therefore unreachable or misconfigured.
Other similar situations may include:
- Non-existent domain (NXDOMAIN)
- Missing MX or A records
- Temporary remote server errors (timeouts, TLS rejection, etc.)
How can these errors be corrected or prevented?
- Verify the domain DNS configuration (A, MX, SPF, DKIM, DMARC).
- Ensure that the Return-Path matches a valid address or domain.
- For automated sending platforms, configure an existing or redirected bounce address.
- In case of DNS errors, check domain resolution using dig, nslookup, or an online DNS analysis tool.
- For technical or no-reply addresses, verify that the domain is properly hosted and that the mail server responds correctly.