What exactly is a soft bounce, and how does it differ from a hard bounce?

A soft bounce is a temporary email delivery failure, meaning the email server successfully received your email but couldn't deliver it to the recipient's inbox at that moment. Reasons include a full mailbox, server downtime, or the message being too large. A hard bounce, conversely, is a permanent failure, like an invalid email address, and means the email will never be delivered to that address.

Why do small ISPs like Windstream and TDS often share spam filtering services?

Smaller ISPs often lack the resources to develop and maintain their own comprehensive spam filtering technologies. To provide email security for their users, they license or outsource solutions from specialized third-party vendors, such as Vade Secure or Cloudmark. This creates a shared security infrastructure across various independent ISPs.

How can I identify which third-party spam filter is blocking my emails?

The diagnostic code in the bounce message often provides clues. For instance, 'VI-1' in the error typically indicates a block by Vade Secure. Researching common error codes associated with the specific ISPs can help you identify the filtering service. You can then use the vendor's sender tools or their abuse/postmaster contact for more information.

What steps should I take if my transactional emails are getting soft bounced as spam?

First, carefully review your email content for anything that might trigger spam filters, such as excessive links, certain keywords, or poor HTML. Next, ensure your email authentication (SPF, DKIM, DMARC) is correctly configured. Finally, contact the ISP's postmaster or the identified third-party filtering service (e.g., Vade Secure) through their sender tools or support channels to inquire about delisting or block reasons.

How often should I check my email sender reputation with these ISPs?

You should continuously monitor your email deliverability metrics, including bounce rates, engagement, and spam complaints, on an ongoing basis. For specific ISPs and their associated third-party filters, it's advisable to check your sender reputation and any blocklist statuses regularly, especially if you observe any unexpected spikes in soft bounces.

Why am I getting soft bounces from Windstream, TDS, CenturyTel, Hughes and Zoom Internet? - Troubleshooting - Email deliverability - Knowledge base

Dealing with email soft bounces can be one of the most frustrating experiences for anyone sending emails, especially when they come from a cluster of seemingly unrelated internet service providers (ISPs) like Windstream, TDS, CenturyTel, Hughes, and Zoom Internet. You might be seeing these bounces for what appears to be no clear reason, leaving you wondering if there's some underlying connection or a specific issue you're missing.

A soft bounce, in essence, is a temporary delivery failure. Unlike a hard bounce which means the email address is permanently invalid, a soft bounce suggests a transient issue, like a full inbox or a server being temporarily down. However, when you receive a diagnostic code explicitly stating "message blocked due to spam content," it points to something more specific than just a temporary server glitch, even if your content hasn't changed.

The key here is understanding why these particular ISPs might be flagging your emails as spam. It often comes down to their shared infrastructure or filtering practices. If you're encountering increased soft bounce rates and want to know how to fix them, a deeper dive into these specific providers is necessary.

Commonalities among these ISPs

While Windstream, TDS, CenturyTel, Hughes, and Zoom Internet are independent entities, they often share common characteristics in their email infrastructure that can lead to similar deliverability challenges. Many smaller to medium-sized ISPs don't have the resources to build and maintain their own sophisticated spam filtering systems from scratch. This often leads them to outsource or license solutions from third-party vendors.

It's common for these providers to use the same security and spam filtering services. For example, you might find several of them relying on well-known email security companies like Vade Secure or Cloudmark. If a particular vendor's filter flags your email, it could simultaneously affect deliverability to multiple ISPs using that same vendor's product. This explains why you might see a sudden increase in soft bounces across several seemingly unrelated domains at once.

This shared dependency means that a change in the filtering logic, an update to a blocklist (or blacklist), or a shift in how the vendor scores sender reputation can have a cascading effect. If one of these filters determines your message content or sender reputation is suspicious, it can result in a `5.7.1` delivery failure across all ISPs utilizing that specific solution, even if your email content hasn't undergone major changes.

Deciphering the 'spam content' bounce

The diagnostic code you're receiving, "`554 5.7.1 [VI-1] Message blocked due to spam content in the message. X-PowerMTA-BounceCategory: spam-related`," provides crucial clues. The `VI-1` often points directly to Vade Secure's filtering system being the culprit. This means that, despite your emails being transactional or having good engagement historically, Vade Secure's algorithms have identified something within your message or sender characteristics as spam-like.

While you might not have intentionally changed your content, minor alterations, new links, or even an increase in sending volume could trigger these sensitive content filters. These filters evaluate many factors, including: the text, image-to-text ratio, links, domain reputation, and IP reputation. For more details on what constitutes a soft bounce, you can refer to Mailbluster's guide on soft bounces. It's a challenging situation because the definition of "spam content" can be broad and constantly evolving from the perspective of an automated filter.

Even with transactional emails, which are generally trusted, these filters can still be triggered. It might not be the direct content as much as a shift in your sender reputation as seen by the filter across its network. Understanding why your emails are going to spam involves a holistic view of your sending practices and reputation.

Strategies for troubleshooting and remediation

When facing these specific soft bounces, your first step should be to investigate your sender reputation across various monitoring services. Even if your internal metrics look good, a specific filter might see something differently. Check if your IP or domain is listed on any common blocklists. While these are soft bounces, a lower-level listing could influence the content filter's decision.

Next, consider making minor content adjustments. Even if you haven't changed the core message, try rephrasing sentences, reducing the number of links, or simplifying your HTML structure. Send a very plain text version of the email to an address on one of the affected domains as a test. If it delivers, you know the issue is indeed related to content. You can also utilize specific sender tools provided by the filtering companies themselves, like the Vade Secure Sender Tool, to inquire about specific blocks.

Finally, direct communication is often necessary. Try to find the postmaster or abuse desk contact for each of these ISPs. While it can be time-consuming, explaining your sending practices and requesting clarification on the block can lead to resolution. For a general understanding of how email blocklists work, consulting comprehensive guides can be helpful.

Maintaining long-term deliverability

Potential content triggers

Wording: Use of certain keywords or phrases frequently associated with spam.
Link structure: Too many links, suspicious link shorteners, or links to domains with low reputation scores.
Image-to-text ratio: Emails with too many images and insufficient text content.
HTML errors: Poorly coded HTML can be interpreted as suspicious by some filters.

Sender reputation factors

IP/domain blocklisting: Even if not a hard block, a listing can lower trust scores.
Spam trap hits: Sending to invalid or old addresses can indicate poor list hygiene.
Low engagement: If recipients rarely open or click your emails, it can signal disinterest or spam.
Complaint rates: High rates of recipients marking your emails as spam.

How to check your IP/domain blocklist status

You can use a reliable blocklist checker to quickly see if your sending IP address or domain is currently listed on any major public blocklists (also known as blacklists). While direct listing might not always be the cause of soft bounces, it can certainly contribute to a lower sender score with ISPs.

Remember that some private blocklists also exist, maintained directly by ISPs or their filtering vendors, which are not publicly accessible.

Example soft bounce error codetext

Status: 5.7.1 (delivery not authorized)
Diagnostic-Code: smtp;554 5.7.1 [VI-1] Message blocked due to spam content in the message.
X-PowerMTA-BounceCategory: spam-related

Maintaining a clean email list is paramount for long-term email deliverability. Regularly remove inactive subscribers and hard-bouncing addresses. This proactive approach signals to ISPs that you are a responsible sender who respects their network and user inboxes.

Beyond list hygiene, consistent sending patterns are crucial. Avoid sending large volumes of emails in infrequent, sudden bursts. A steady cadence helps build and maintain a positive reputation with receiving servers. ISPs appreciate predictable senders, as erratic behavior can often be a red flag for spam.

Finally, strong email authentication (SPF, DKIM, and DMARC) is non-negotiable. These protocols verify that you are who you say you are, significantly increasing trust with ISPs and their third-party filters. If you need a refresher, check out our simple guide to DMARC, SPF, and DKIM. Proper authentication provides the foundational trust needed for successful inbox placement, especially with providers that employ stringent spam content filtering.

Views from the trenches

Best practices

Actively monitor all bounce types, not just hard bounces. Soft bounces can provide early warnings.

Segment your email lists by ISP where possible, to identify unique deliverability challenges.

Engage directly with ISP postmaster teams. Building relationships can aid in quicker resolutions.

Common pitfalls

Assuming no content changes means no content issues; even minor shifts can trigger filters.

Ignoring soft bounces, as they can escalate into hard blocks or impact sender reputation.

Failing to implement and monitor DMARC, SPF, and DKIM for consistent authentication.

Expert tips

If the diagnostic code points to a specific vendor like Vade Secure or Cloudmark, prioritize using their sender tools or abuse contact forms directly.

Consider a phased rollout for new email templates or content to observe deliverability impact before a full send.

Transactional emails often have a higher expectation of deliverability. Any block is a critical signal to investigate immediately.

Expert view

Expert from Email Geeks says that while the specific ISPs are different, the VI-1 code strongly suggests Vade Secure's filtering system is at play for several of them. It's a common outsourced solution.

2022-10-28 - Email Geeks

Marketer view

Marketer from Email Geeks says they've encountered similar issues where multiple smaller ISPs suddenly block due to perceived spam content, even for trusted transactional senders. It usually points to a shared underlying filter or a reputation trigger.

2022-10-28 - Email Geeks

Navigating ISP filtering challenges

Experiencing soft bounces from ISPs like Windstream, TDS, CenturyTel, Hughes, and Zoom Internet, particularly with "spam content" messages, often indicates a common filtering service at play behind the scenes. These temporary blocks require a combination of diligent monitoring, technical review, and proactive engagement with the relevant postmaster teams or their third-party filtering vendors. By understanding the common characteristics of these ISPs and their filtering choices, you can better diagnose and resolve your deliverability challenges, ensuring your important emails reach their intended recipients.