You have received an email from “Mail Delivery System” with the subject “Undelivered Mail Returned to Sender” and the sender is from a paubox.com email address. This document will explain why you have received the email and provide resources to better understand the content of the email.
Email Purpose
When Paubox Encrypted Email attempts to deliver outbound email if the recipient’s email server rejects the email Paubox will notify the sender that the email was not delivered.
Paubox notifies the sender in two ways:
- Paubox Dashboard, Encrypted Email, Mail Log, Status Column
- Email to sender from "Mail Delivery System" with subject "Undelivered Mail Returned to Sender".
Following is an example of how the email will be formatted and information to help interpret the contents of the email.
Email Example
From: Mail Delivery System
Subject: Undelivered Mail Returned to Sender
Message body similar to:
This is the mail system at host <paubox_server>.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<recipient_address>: host <recipient_email_server>[<ip_address>] said:
<status_code> <status_code_description>
---------- Forwarded message ----------
Portions of the message are variable, the following table describes the type of information you can expect to see in each variable part of the message.
| <paubox_server> | A server name ending in paubox.com |
| <recipient_address> | The intended recipient of the email which was returned (not delivered). |
| <recipient_email_server> | The email server that returned the email. |
| <ip_address> | IP Address of the server that returned the email. |
| <status_code> | The SMTP status code. |
| <status_code_description> |
A description of what the status code means. |
Interpreting SMTP Status Codes
Typically the recipient server will return a plain text description of an error state in addition to the numeric SMTP Status Codes however in cases where a description is not provided the following resources can be used to translate the numbers into descriptive text based on SMTP standards.
An SMTP status code is typically structured one of two ways.
- ###
- ### #.#.#
In both cases the first 3 numbers are the basic SMTP Status Code. If they are present the additional numbers formatted like #.#.# are called the Enhanced Status Code.
Some common status codes:
- 513: Email address is misspelled or there is an error with authentication
- 550 It usually defines a non-existent email address on the recipient side
- 553 Requested action not taken – Mailbox name invalid (incorrect email address)
RFC 821 4.2.2. Numeric Order List of Reply Codes
RFC 3463 Appendix A - Collected Status Codes