Sisimai: Mail Analyzing Interface

When Does Email Bounce?

Last Updated: 6th June, 2024

When an email you send is returned due to an error, it is called a bounce. The diagram below illustrates the potential points at which a bounce can occur during the delivery process of an email from your sender's mailbox to the recipient's mailbox.

There are three main stages at which a bounce can occur:
1. Pre-SMTP Connection, 2. During SMTP Session, and 3. During Forwarding. In any of these scenarios, a bounce email will be sent to the email address specified in the Return-Path: header of the original email. This bounce email will contain information about the error that caused the delivery failure.

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1

1. Pre-SMTP Connection (5%)

An error 1 in the diagram below illustrates errors that can occur before an SMTP connection is established. These errors account for approximately 5% of all bounces. The primary causes include:

Non-existent Domain in Email Address Host:
The domain name specified in the email address's host part (the part after the @ symbol) does not exist.
Unresolvable Host in Email Address:
The host part of the email address cannot be resolved using DNS (neither MX nor A records are found).
Null MX Record:
The host part of the email address has a Null MX record.
Unreachable Destination Mail Server IP:
The IP address of the destination mail server, obtained through DNS resolution, is unreachable.
Failed SMTP Connection to Destination Mail Server:
An SMTP connection to the destination mail server cannot be established (no response or connection is refused).
Network Issues Between Sender's and Recipient's Mail Servers:
Network connectivity problems between the sender's and recipient's mail servers.
Unresponsive Nameserver:
The nameserver used by the sender's mail server is not responding.

When Sisimai decodes bounce emails caused by connection errors, it categorizes them based on the specific error and the bounce message. The following categories and their corresponding bounce reasons are common:

Expired:
Emails that could not be delivered within a specified timeframe are categorized as Expired.
HostUnknown:
Emails that bounce due to an invalid or non-existent domain in the recipient's email address are categorized as HostUnknown. This suggests that the domain name specified in the email address does not exist or is not properly configured in DNS.
NotAccept:
Emails that bounce due to a Null MX record for the recipient's domain are categorized as NotAccept. A Null MX record indicates that the recipient's mail server is configured to not accept any email.


Bounce emails resulting from errors occurring before the SMTP connection are generated by the sending server sender.example.jp, as illustrated in the diagram.

DNS Lookup Failed

Among the most common causes of pre-SMTP connection error bounces are issues with the authoritative DNS content server for the recipient's email address domain (the part after the @ symbol). In some cases, the DNS server referenced by the sender's mail server may also be malfunctioning. When Sisimai decodes bounce emails returned due to such reasons, the bounce reason is categorized as NetworkError.

No Response or Network Error

In most cases, these bounces occur due to the recipient's mail server being down or their network's firewall blocking inbound SMTP connections.

However, it is also possible that the sender's network firewall is restricting outbound SMTP connections. In such cases, OP25B (Outbound Port 25 Blocking) implemented by the ISP (Internet Service Provider) might be preventing the connection. Regardless of the cause, these emails are categorized as Expired by Sisimai's decoding, as they are retried until the network recovers and eventually time out.

Non-Existent Domain

If the recipient's email address domain does not exist or neither A nor MX records are set, Sisimai will determine the bounce reason as HostUnknown (Hard Bounce). Therefore, please remove the corresponding email address from the sender list as soon as possible.

In rare cases, if a third party has registered that domain, and has configured their mail server to receive all email addresses for that domain, there is a risk of unintentional matching with the subscriber names and email addresses listed at the beginning of the email newsletter, leading to potential privacy concerns.

2

During SMTP Session (25%)

An error 2 in the diagram below illustrates bounces that occur due to errors during the SMTP session after establishing a connection to the recipient's mail server. These bounces account for approximately 25% of all bounces.

In the past, the majority of bounces were caused by errors during the SMTP session. However, since the phased implementation of bulk sender guidelines by Gmail and Yahoo! starting in February 2024, bounces caused by forwarding emails, particularly those related to domain authentication errors, have become the dominant type.

When Sisimai decodes bounce emails occurred during SMTP session, the bounce reason is determined to be related to the recipient's email address or mailbox, such as UserUnknown, MailboxFull, or MesgTooBig.

In recent years, there has been an increase in bounces due to stricter domain authentication requirements, resulting in AuthFailure bounces. Additionally, bounces related to the reputation of the sender's IP address or domain name can be categorized as BadReputation.

Other common bounce reasons include RequirePTR, which indicates a requirement for proper reverse DNS lookup of the sender's IP address, and NotCompliantRFC, which signifies non-compliance with RFC 5322 standards in the email header.

Similar to the bounces described in the previous section(1), bounce emails resulting from errors during the SMTP session are generated by the sending server, represented as sender.example.jp in the diagram.

3

During Forwarding (70%)

In many cases, emails sent to an organization's email address are forwarded to Gmail or Outlook to receive notification on smartphones. However, since the phased implementation of sender guidelines by Gmail and Yahoo! starting in February 2024, forwarded emails have become a major contributor to bounces, with a significant increase compared to previous years. According to our observations over the past few years, bounces caused by forwarding errors have accounted for approximately 70% of all bounces.

Specifically, when forwarding emails received at mx.example.com in the diagram, SPF hard failures or DKIM signature verification failures can occur due to partial rewriting of the Subject: header or body (which often happens with mailing lists), resulting in bounces due to domain authentication failures.

Unlike bounces described in cases 1 (Pre-SMTP connection) and 2 (During SMTP Session) above, bounces in this case are generated by mx.example.com in the diagram and sent to the email address specified in the Return-Path: header.

When Sisimai decodes bounce emails caused by domain authentication errors during email forwarding, the bounce reason is determined to be AuthFailure. If the forwarding server (mx.example.com in the diagram above) supports ARC, ARC-related headers will be added to the email as a technically verifiable record of the fact that "SPF, DKIM, and DMARC were all OK when we received it," which may help the forwarding destination accept the email.

Alternatively, if the forwarding server (mx.example.com in the diagram above) IP address has a low reputation and the connection is refused, the bounce reason will be determined to be BadReputation.

In recent years, domain authentication-related errors, particularly those related to DMARC and DKIM, have become more prominent. However, bounces caused by nonexistent recipient email addresses (UserUnknown) and neglected recipient mailboxes (MailboxFull) are still occurring.

Regardless of the bounce reason, if you are sending a significant volume of emails, you should consider removing recipient email addresses that generate errors during email forwarding from your subscribers list.