One element of email that flies under the radar is the bounce domain - what it is, why it matters, and how it relates to other email authentication elements like SPF, DKIM, and DMARC. Let’s break it down step by step in simple terms.
What is a Bounce Domain?
When you send an email, there’s more going on than what meets the eye. While your recipients see the visible “From” address - the sender name and email address in their inbox - there’s also a hidden technical address working behind the scenes. This is the bounce domain.
This term refers to the 5321.From, a technical address that serves as the return place for bounces (non-delivery notifications) if an email can’t be delivered.
Proper configuration of the bounce domain is crucial for successful email delivery. Mailbox providers perform checks on this domain as part of their anti-spam and email authentication measures. If the bounce domain is not correctly configured, some mailbox providers may reject the message or mark it as suspicious. This makes it essential to ensure that the bounce domain is properly set up.
The Two Sides of the Sender
To understand the bounce domain’s role, it’s important to differentiate between two key email domains:
5322.From: This is the visible sender address that recipients see in their inbox. Also known as Header From, Visible From, Sender From, hfrom, from or FROM. In Emarsys we call it Sender Domain/ Sender From.
5321.From: The technical address used during email transit to return bounces. Also known by many names such as Bounce domain, SPF domain, Return-Path, Envelope Sender, SMTP from, Mail from, mfrom, mailed-by or Envelope From. In Emarsys we call it Bounce Domain/ Bounce Address.
Below we will use Emarsys terms, however, these numbers, 5321 and 5322, aren’t random. They refer to specific RFC (Request for Comments) documents that define how email systems work. RFCs are technical standards established to ensure consistent protocols across the internet.
The Relationship Between Bounce Domain and Sender Domain
When it comes to email authentication and deliverability, the relationship between these two domains - bounce domain and sender domain - is important. This relationship is evaluated through alignment.
- Strict alignment: the bounce domain must exactly match the sender domain.
- Relaxed alignment: the bounce domain must belong to the same organizational domain as the sender domain (for example, bounces.news.example.com aligns with news.example.com).
- Not aligned: the domains are unrelated.
Despite the common belief that SPF is checked against the sender domain, mailbox providers actually check SPF against the bounce domain. However, it’s still important for the sender domain to have an SPF record for added protection, and also some mail servers may mistakenly check SPF against the sender domain. This is why we still recommend adding an SPF record to the sender domain, even though it’s not the primary domain used for SPF validation by major providers.
How Alignment Works
Alignment plays a central role in ensuring that emails pass DMARC (Domain-based Message Authentication, Reporting, and Conformance) policies. To pass DMARC check, either:
- the sender domain must align with the bounce domain (SPF alignment); OR
- the sender domain must align with the DKIM domain (DKIM alignment).
At Emarsys, DKIM is always strictly aligned with the sender domains, which complies with the current requirements of Google, Yahoo, and other major mailbox providers. This means that even if SPF alignment is not strict or is entirely absent, DMARC will still pass because of the mandatory DKIM alignment.
Bounce Domains in Emarsys: Options and Configurations
While DKIM alignment is mandatory and strictly enforced at Emarsys, we provide several options for configuring the bounce domain based on your specific needs:
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Non-aligned: in this setup, a shared bounce domain is used across multiple sender domains and often multiple Emarsys customers. This configuration does not align the bounce domain with the sender domain.
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Relaxed alignment: in this setup, the bounce domain is a part of the same organizational domain as the sender domain, allowing a more relaxed match. This is our current default DNS setup.
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Strict alignment: for customers who require a strict alignment, we can configure the bounce domain to exactly match the sender domain if feasible.
It’s important to note that each of these configurations comply with the requirements of mailbox providers. As long as DKIM alignment is maintained (which it always is at Emarsys), strict SPF alignment is not required to pass DMARC or to comply with recent Yahoo and Google requirements.
A question we often hear is about seeing a 0% SPF success rate in Google Postmaster Tools. This happens because Google checks not just whether SPF is valid, but also whether it’s aligned with the sender domain. The good news is that Google only requires either DKIM or SPF to be aligned—not both. So, if you see a 100% success rate for DKIM and DMARC, your setup is fully compliant. For more details on this, you can refer to our article DKIM Basics.
Current Limitations in Emarsys Bounce Domain Setup
While Emarsys offers robust solutions for bounce domains, there are some limitations.
Currently, we can only assign one bounce domain per sending IP configuration. If multiple different domains share the same sending IPs, only one bounce domain can be used. Thus, only one sender domain among the multiple ones can be aligned.
For customers who use separate domains for marketing and transactional emails, alignment can be configured for one sender domain per each IP setup (marketing and transactional IPs).
We are actively working on enhancing our platform to provide relaxed bounce domain alignment for each sender domain, even when multiple domains share the same IP configuration.
Takeaways
Bounce domain alignment contributes to building trust with mailbox providers by ensuring consistency between the bounce and sender domains. While SPF alignment is not mandatory as long as DKIM alignment is in place, maintaining a well-configured bounce domain setup can enhance your email-sending reputation and improve deliverability.
At Emarsys, we prioritize compliance with industry standards and the requirements of major mailbox providers.
By continuing to innovate and enhance our platform, we ensure that you have the tools and flexibility to meet your email-sending goals while staying ahead of evolving industry requirements.