Why does bounce data from Drift Email differ from Hubspot's bounce data?
Michael Ko
Co-founder & CEO, Suped
Published 16 May 2025
Updated 19 Aug 2025
8 min read
Email marketing is full of metrics, and bounce rate is one of the most critical. It tells you whether your emails are actually reaching recipients' inboxes or if they're being rejected. However, it's not uncommon to see different bounce figures when comparing data from various email platforms or tools.
When comparing bounce data from a primary Email Service Provider (ESP) like HubSpot with a third-party tool such as Drift Email, you might notice discrepancies. This can be puzzling, especially when you're trying to understand your true deliverability performance and ensure your customer service team is notified about critical delivery failures. These differences are often a result of how each platform handles and categorizes email bounce types, as well as their role in the email sending process.
Understanding these nuances is key to accurately interpreting your email data and taking appropriate action. It helps you avoid confusion and ensures you're working with reliable information to maintain a healthy email sending reputation.
Understanding bounce detection mechanisms
To grasp why bounce data differs, we first need to understand the fundamental ways email bounces are communicated. There are two primary types of bounce notifications: in-band and out-of-band.
In-band bounces: These are immediate rejections that happen during the initial SMTP (Simple Mail Transfer Protocol) conversation between the sending server and the recipient's mail server. The sending server receives an error code (like a 550 mailbox not found) directly. Primary ESPs like HubSpot are designed to detect and process these in real-time, preventing further sending to invalid addresses.
Out-of-band bounces: These occur after the initial delivery attempt. The recipient's server might accept the email but later send a separate non-delivery report (NDR) or a machine-generated auto-reply indicating a permanent or temporary failure. These notifications are sent to the return-path address specified in the email's headers.
HubSpot, as your primary ESP, is responsible for the actual sending of your emails. This means it has direct access to the most accurate bounce information, especially in-band bounces. It immediately records these rejections and updates the contact's status. For example, a permanent failure, often called a hard bounce, will lead HubSpot to automatically stop sending to that address to protect your sender reputation. HubSpot classifies bounces into categories such as hard bounces, soft bounces, and pending bounces. This is why its data is generally considered the source of truth for its own sends.
Third-party tools, unless they are also sending emails, often rely on different methods to gather bounce information. They might monitor incoming replies, parse automated messages, or have their own integrations that interpret data. This can lead to discrepancies because they are not directly involved in the initial SMTP transaction that generates in-band bounces.
Differences in reporting and interpretation
The core of the discrepancy often lies in how each platform is designed to handle bounce notifications. HubSpot (and most primary ESPs) have robust systems in place to manage bounces directly from the mail server. They use a specific return-path or bounce address for their asynchronous bounce handler. This means that if an email hard bounces (or soft bounces for too long), HubSpot itself receives that notification and updates its records accordingly.
Drift Email, on the other hand, is primarily designed for conversational marketing and collecting replies. While it might encounter messages that look like bounces (e.g., automated out-of-office replies or undeliverable messages), these are often out-of-band notifications. If HubSpot successfully delivered the email, it means the recipient's server accepted it. Any subsequent message that Drift interprets as a bounce might be a later rejection, a message caught by a corporate spam filter, or simply a different type of automated response that isn't a true bounce from the primary ESP's perspective.
HubSpot's bounce handling
Direct SMTP communication: HubSpot is involved in the initial handshake with recipient mail servers, allowing it to capture in-band bounces accurately.
Return-path monitoring: Uses its own dedicated return-path for receiving bounce messages, which are then processed internally.
Sender reputation management: Automatically suppresses future sends to hard-bounced addresses to protect your domain reputation.
Reliability of data
When an email is sent through HubSpot, if it truly bounces, HubSpot will know and record it. Its bounce data is generally reliable for emails sent through its platform.
Drift email's bounce handling
Reply monitoring: Drift may interpret automated replies, such as mailbox full or address no longer exists, as a bounce even if the original email was technically delivered.
Secondary signal interpretation: Might rely on its own logic or third-party integrations to infer deliverability issues, which can sometimes differ from direct SMTP feedback.
No direct involvement in send: Unless Drift is also sending the emails, its bounce data isn't from the initial delivery attempt.
Reliability of data
If HubSpot reports an email as delivered, but Drift reports it as bounced, the HubSpot data is likely more accurate regarding the initial delivery status.
This difference in how platforms handle bounces can impact your overall understanding of email deliverability. Relying solely on a secondary tool's bounce data (or blocklist reporting) might lead you to misinterpret your performance, potentially causing you to remove valid contacts from your lists or unnecessarily adjust your sending strategy.
If HubSpot registers a successful delivery, but Drift indicates a bounce, it's crucial to understand the context. The HubSpot data is the authoritative one for deliverability directly from your sending infrastructure. The information from Drift might point to an issue post-delivery, such as an automatic vacation responder or a corporate mail gateway rejecting the email after accepting it.
Impact on deliverability and data accuracy
Discrepancies in bounce data can be frustrating, but they highlight the importance of understanding the underlying mechanisms. Here's how these differences can impact your deliverability and what steps you can take.
First, relying on inaccurate bounce data (or blocklist information) can lead to poor decision-making regarding your contact lists. If Drift marks an email as bounced that HubSpot successfully delivered, removing that contact based on the Drift data might mean losing a potentially engaged subscriber. Conversely, if you're not capturing true hard bounces, you could be damaging your sender reputation.
Secondly, consistent monitoring of your bounce rates in your primary ESP is crucial for maintaining good email deliverability. A high bounce rate can lead to your emails being flagged as spam or even your IP address being placed on an email blocklist (or blacklist). Always trust the deliverability metrics provided by the platform that actually sends your emails first and foremost.
Strategies for reconciling data
To effectively manage your email campaigns and ensure accurate reporting, consider the following strategies.
Prioritize primary ESP data: HubSpot's bounce data should be your primary source of truth for email deliverability. It has the most direct feedback from mail servers. Regularly review your bounce data within HubSpot.
Understand bounce types: Familiarize yourself with the differences between hard and soft bounces. Hard bounces indicate permanent errors and should lead to immediate removal from your lists. Soft bounces are temporary and may resolve on their own, but persistent soft bounces might require further investigation.
Use secondary tools for insights, not primary data: Tools like Drift can provide valuable insights into engagement or automated replies, but their bounce interpretations should be viewed as supplementary, not definitive, especially if they contradict your ESP.
Integrating these practices will give you a clearer and more accurate picture of your email deliverability. Remember, the goal is to get your messages into the inbox, and accurate bounce data is a critical part of that process.
Focus on the primary ESP
Your Email Service Provider (HubSpot in this case) is your authoritative source for bounce data. They directly interact with recipient mail servers and accurately capture in-band bounces and manage return-path notifications. Trust their reporting first.
Monitor for unexpected patterns
While discrepancies are expected, a sudden or significant shift in bounce rates reported by either platform could indicate a larger issue, such as a reputation problem or a blocklist listing on your sending domain or IP. Investigate these anomalies promptly.
Views from the trenches
Best practices
Always prioritize the bounce data reported by your primary Email Service Provider (ESP), like HubSpot, as it has the most accurate view of delivery outcomes.
Regularly review your ESP's bounce reports and categorize them into hard bounces (permanent failures) and soft bounces (temporary issues) for proper list hygiene.
Implement email validation processes before sending to reduce hard bounces and protect your sender reputation.
Configure your email sending to align with DMARC, DKIM, and SPF best practices to improve deliverability and reduce rejections.
Common pitfalls
Misinterpreting automated replies or out-of-band notifications from secondary tools as true delivery bounces, leading to incorrect contact removal.
Failing to regularly clean your email lists of hard bounces, which can negatively impact your sender reputation and lead to blocklisting.
Not understanding the difference between in-band and out-of-band bounce reporting, causing confusion in data analysis.
Overreacting to minor discrepancies in bounce rates between different platforms without understanding their reporting methodologies.
Expert tips
The return-path (bounce address) is where mail servers send non-delivery reports (NDRs). Your primary ESP handles these directly, so third-party tools typically won't see them.
Most legitimate email rejections happen during the initial SMTP connection (in-band) and are immediately known by the sending ESP.
If a third-party tool reports a bounce but your primary ESP shows delivered, the primary ESP's data is more accurate for the initial delivery status.
Consider the purpose of each tool: ESPs manage delivery, while other tools might focus on post-delivery interactions like replies or engagement.
Expert view
Expert from Email Geeks says most bounces likely occur in-band at the time of delivery within HubSpot, while Drift likely catches out-of-band replies or automated messages.
2023-12-15 - Email Geeks
Expert view
Expert from Email Geeks says almost all rejections will be in-band and will not involve the return path. HubSpot sends mail with a return path set to their asynchronous bounce handler, and no third party will see those.
2023-12-15 - Email Geeks
Navigating data discrepancies
The differences in bounce data between Drift Email and HubSpot are common and stem from their distinct roles in the email ecosystem. HubSpot, as your primary ESP, has the most accurate and real-time information about whether an email was successfully delivered or rejected at the SMTP level. Any perceived bounces reported by Drift are likely out-of-band notifications or interpretations of automated replies.
By understanding these distinctions, you can make more informed decisions about your email lists and deliverability strategies. Always rely on your ESP for definitive bounce data, and use secondary tools to gather additional insights into recipient engagement and automated responses.
HubSpot with a third-party tool such as 
Drift Email, you might notice discrepancies. This can be puzzling, especially when you're trying to understand your true deliverability performance and ensure your customer service team is notified about critical delivery failures. These differences are often a result of how each platform handles and categorizes email bounce types, as well as their role in the email sending process.
