- mailer-daemon@gmx.de - mailer-daemon@yahoo.com - mailer-daemon@otherdomains.com

You can see that all the mailboxes listed above have the name "mailer-daemon" before the @ symbol. This is done intentionally to make it easier to understand that this is a useful delivery failure notification email.

These delivery failure notifications (reports) are sent by a special robot program "Mailer-Daemon" (mailer demon).

The Mailer-Daemon program is installed on the recipient's email server. If your sent email is not delivered for some reason, the Mailer-Daemon program is activated, which determines the reason for non-delivery and sends you a notification email with an explanation of this reason.

There can be many reasons for email delivery failures:

User not found:

  • User not found
  • User unknown
  • No such user here
  • Unrouteable address
  • Invalid mailbox
  • Mailbox unavailable

No free space in mailbox to receive emails:

  • Account is full
  • Quota exceeded
  • User has exhausted allowed storage space
  • Error writing message: Disk quota exceeded

Restriction on receiving emails with large size:

For example, if your email is very large and takes up 8 MB, and the server is only ready to accept emails up to 4 MB in size, the recipient's server will reject such emails. With the help of mailer-daemon, it will inform you about this.

Recipient server does not accept mail:

The server may have a restriction configured on receiving emails, or email reception may not be configured at all.

More detailed information about email delivery failure can be viewed in the notification email itself.

Why Am I Receiving a Large Number of Emails from mailer-daemon?

This can happen when you do a mass email campaign to your customers. If you haven't sent emails to your database for a long time, many of your customers' mailboxes have probably ceased to exist or may have become full. Therefore, Mailer-Daemon will send you a notification for each such case.

FBL, RUA, RUF Reports

Let's understand:

FBL (Feedback Loop) report is a feedback message sent by email providers (such as Gmail, Yahoo, Mail.ru, Outlook, and others) to email senders about their message being marked as spam by the recipient or a complaint being filed against it.

This report usually contains information about the specific email marked as spam, including the message identifier and recipient information (for example, recipient IP address or email address).

FBL reports provide senders with valuable information about which of their emails are considered spam by recipients, which helps them analyze and improve their campaigns to better meet recipient expectations and avoid being marked as "spam" by them. In addition, FBL reports allow quick response to possible sender reputation issues and manage mailing lists.

RUA (Aggregate Reporting URI) and RUF (Feedback Reporting URI) reports are part of the DMARC (Domain-based Message Authentication, Reporting, and Conformance) system, which is used to improve email security and protect against spam and phishing.

  1. RUA (Aggregate Reporting URI) reports are aggregated statistics about email delivery from your domain, generated by email providers and services. They contain information about the number of emails sent from your domain, how many were successfully delivered, how many were rejected (for example, due to SPF or DKIM errors), and how many were marked as spam. These reports allow tracking and analyzing the effectiveness of your email security policy and taking measures to improve email delivery.
  2. RUF (Feedback Reporting URI) reports are detailed feedback on email delivery from your domain, which includes information about each specific email sent from your domain and marked as spam or rejected for some other reason. RUF reports allow analyzing specific cases, finding out the reasons why emails were marked as spam, and taking measures to improve delivery and protect your domain from abuse.

Thus, RUA reports provide general statistics on email delivery from your domain, while RUF reports provide detailed information about each specific email that was marked as spam or rejected for some other reason.

  1. Mailer-Daemon — notifications about delivery failure of a specific email.
  2. FBL — notifications about complaints and emails being marked as spam.
  3. RUA — aggregated statistics on email delivery from your domain, indicating how many were successfully delivered, how many were rejected (for example, due to SPF or DKIM errors), and how many were marked as spam.
  4. RUF — detailed feedback on email delivery from your domain, which includes information about each specific email sent from your domain and marked as spam or rejected for some other reason.

Where Do Email Providers Get Your Email Addresses to Send Notifications?

  1. Mailer-Daemon. The email address for receiving notifications is taken by email providers (Gmail, Yahoo, Yandex, Mail.ru, etc.) from the email itself and equals the sender address.
  2. FBL. The email address for receiving FBL reports is most often specified independently through special forms. For example, Yahoo previously had a special CFL form for receiving complaints from recipients of your emails. Also, the address can be specified in a special dashboard, for example, Mail.ru in Postmaster.mail.ru, Outlook in Postmaster Outlook, or in Yahoo Sender Hub. In addition, the address can be taken from the email itself or determined by NS record.
  3. RUA and RUF. Email addresses are specified in the DMARC DNS record itself. You can learn about how to configure a DMARC record from our Knowledge Base article.