Why did my email campaign have high soft bounces during US Thanksgiving week?

Key findings

• Significant Rate: A 20% soft bounce rate (862,548 out of 4.3 million) is generally considered surprisingly high, even during a holiday week. What is a reasonable soft bounce tolerance is an important question for ongoing deliverability.
• Gmail Dominance: The vast majority of soft bounces originating from Gmail addresses is a key indicator, especially given Gmail's specific handling of incoming mail.
• Bounce Reasons: Primary bounce reasons include 554 5.4.7 [internal] exceeded max time without delivery and 550 [internal] The message is an auto-reply/vacation mail. The first indicates deferrals from your own MTA, not the receiver's outright block, pointing to a temporary issue or reputation challenge on the sender's side.
• Holiday Impact: While increased email volume during holiday periods like Thanksgiving is a known factor, it typically leads to temporary deferrals rather than outright high soft bounces from legitimate lists, unless combined with other issues like poor sender reputation.
• Additional Signals: A concurrent spike in spam reports and email truncation (due to size) further suggests content or list hygiene issues contributing to deliverability challenges, not just holiday traffic.

Key considerations

• Bounce Categorization: Be aware that some ESPs might categorize block bounces as soft bounces due to internal configuration, obscuring the true nature of the bounce.
• Detailed Bounce Analysis: Thoroughly analyze bounce logs (exporting and filtering by domain and bounce code) to pinpoint specific issues, such as recipients being over quota, which can occur with high traffic. For example, Google's documentation mentions 452 4.2.2 over quota errors.
• List Hygiene: Regularly cleaning your email lists, especially those derived from app signups that might not have recent engagement, can significantly reduce soft bounces and improve overall deliverability.
• Engagement Monitoring: Mailing lists infrequently can lead to higher bounces as recipients may not remember opting in, affecting domain reputation. Consistent, good quality mailings help establish steady volume expectations with ISPs.
• ESP Logs and GPT: Collaborate with your ESP to review exact deferral messages in logs. Additionally, monitor Google Postmaster Tools for delivery failures or reputation shifts during the incident period to confirm whether the issue was temporary or reputation-related. Even if reputation remains high, spikes in delivery failures are worth investigating.

Key opinions

• High Rate is Unusual: Most marketers agree that a 20% soft bounce rate for a monthly newsletter is surprisingly high, regardless of the holiday season, as ESPs typically filter out auto-replies or OOO messages from bounce statistics.
• ESP Classification Issues: Some ESPs, like Salesforce Marketing Cloud, might categorize what are effectively block bounces as soft bounces if the account isn't configured properly to distinguish them.
• List Age and Activity: Sending to an old or inactive list, even a regularly mailed one, can lead to higher bounces, especially if recipients no longer remember opting in or have abandoned the email address.
• Content Truncation: An email being truncated in Gmail (typically due to excessive size) can indirectly affect engagement and perception, potentially contributing to negative signals that lead to deferrals.
• BFCM Volume Impact: The sheer volume of emails during the Black Friday Cyber Monday (BFCM) period might push some recipients' inboxes over their storage limits, leading to temporary bounces. Email best practices often emphasize warming up IPs and managing volume.

Key considerations

• Drill Down into Bounces: Marketers should always export bounce data, filter by receiver domain, and analyze bounce codes to identify specific patterns or problem areas, rather than relying solely on aggregate soft bounce numbers.
• Unique Bounces: It's critical to determine if the reported soft bounces represent unique email addresses or include duplicate bounces for the same address, which can inflate numbers. Ensure your ESP properly tracks unique bounces.
• List Quality Over Quantity: Even for monthly newsletters, consistent list maintenance is paramount. Regular mailings (with good engagement) can help passively clean a list, but active segmentation and removal of unengaged subscribers are also key to prevent future deliverability issues.
• Monitor Spam Complaints: A spike in spam reports during the same period indicates that recipients are actively marking your emails as spam, which can severely impact sender reputation and lead to more deferrals or blocklisting.

Key opinions

• MTA-Generated Bounces: The 554 5.4.7 [internal] exceeded max time without delivery bounce message typically originates from the sender's Mail Transfer Agent (MTA), not the recipient's server. This indicates that the emails were deferred by Gmail, and your MTA eventually gave up trying to deliver them.
• Deferrals vs. Blocks: Such deferrals can stem from temporary technical failures on the recipient side or, more commonly, from reputation-related problems on the sender's end (e.g., lower sending reputation causing Gmail to throttle incoming mail). This differs from an outright block or blocklist event.
• Gmail Policy Changes: Patterns observed suggest Gmail made changes to bounce messages around November 20th, leading to a general increase in 'storage space' bounces. This indicates potential shifts in how Gmail handles certain overload scenarios.
• Volume Spike as a Factor: If Google Postmaster Tools (GPT) shows only a small, subsiding spike in delivery failures and consistent high IP/domain reputation, the high soft bounce rate is likely attributable to the sheer volume spike during the holiday week, rather than a sustained reputation hit.

Key considerations

• ESP Log Investigation: It is paramount to ask your Email Service Provider (ESP) for the exact deferral messages and logs from Gmail. These detailed logs provide critical insights into why Gmail temporarily rejected the emails.
• Google Postmaster Tools: Regularly consult Google Postmaster Tools (GPT). Pay close attention to delivery errors and reputation dashboards to correlate soft bounce spikes with any shifts in your sender reputation or specific error categories.
• Message Content Review: Reviewing email content for elements that could trigger spam filters or truncation (like excessive size) is important, as these can indirectly lead to deferrals and soft bounces by negatively affecting user experience or sender reputation signals. Understanding bounce types can help frame the investigation.
• List Quality is Key: Even for app signups, continuous monitoring of engagement and suppression of inactive subscribers is vital. A high volume of unengaged or problematic recipients can trigger ISP throttling and lead to soft bounces, especially during peak sending times.

Key findings

• Temporary vs. Permanent: SMTP standards define 4xx series codes for temporary failures (soft bounces) and 5xx series codes for permanent failures (hard bounces). However, some 550 codes, when accompanied by specific internal messages (e.g., auto-reply), can be treated as temporary by ESPs.
• Over Quota: The 452 4.2.2 SMTP code specifically indicates that the recipient's mailbox is over quota, a common temporary bounce reason, especially during high email volume events.
• Deferred Delivery: Messages like 554 5.4.7 [internal] exceeded max time without delivery mean the sending server (MTA) attempted delivery multiple times but couldn't complete it within a defined timeframe, indicating that the recipient server was actively deferring the mail for various reasons, including reputation or temporary overload.
• ISP Throttling: Internet Service Providers (ISPs) like Gmail employ sophisticated throttling mechanisms. During periods of high inbound email traffic (like Thanksgiving/BFCM), or if a sender's reputation is marginal, ISPs may temporarily defer incoming mail to manage server load and filter out suspicious traffic.
• Sender Reputation's Role: Documentation consistently highlights that sender reputation (based on factors like spam complaints, bounce rates, and engagement) heavily influences an ISP's decision to accept, defer, or reject mail. Even temporary issues can be amplified if reputation is not stellar.

Key considerations

• SMTP Error Codes: Referencing the full range of SMTP error codes (e.g., in RFCs) helps distinguish between temporary and permanent delivery issues. While an ESP may classify everything as a 'soft bounce', the underlying SMTP code provides critical diagnostic information.
• Feedback Loops and Postmaster Tools: ISPs provide feedback loops and postmaster tools (like Google Postmaster Tools) to give senders insights into their deliverability. These platforms are essential for understanding spam complaint rates, IP/domain reputation, and specific delivery errors reported by the ISP.
• Content Best Practices: Documentation often emphasizes keeping email size reasonable (e.g., under 102KB for Gmail) to avoid truncation, which can negatively impact user experience and potentially lead to lower engagement and increased spam reports.
• List Management: Maintaining a clean and engaged subscriber list is foundational. Documentation on email marketing best practices (such as Mailchimp's guide on bounces) consistently advises removing unengaged or invalid addresses to improve deliverability metrics.