How do SMS spam complaints impact email domain reputation and Razor blacklists?
Michael Ko
Co-founder & CEO, Suped
Published 29 Jun 2025
Updated 16 Aug 2025
7 min read
When we talk about email deliverability, the focus is often on factors directly related to email sending practices: things like DMARC, SPF, DKIM, content, and list hygiene. It's a common assumption that if you're not sending problematic emails, your email domain reputation should be safe. However, there's a less obvious connection that can profoundly affect your email domain's standing, and it involves SMS spam complaints and a particular blocklist known as Razor.
The surprising reality is that negative signals from one communication channel can spill over and harm your reputation on another. For instance, if your domain is used in SMS messages that recipients report as spam, this can have ripple effects, potentially leading to your domain being listed on certain blocklists (or blacklists) that also influence email systems. This often catches senders by surprise.
I've seen situations where a domain, primarily used for email, suddenly faces deliverability issues due to complaints originating from SMS campaigns where the domain was merely mentioned. This highlights the interconnected nature of digital communication channels and the comprehensive approach needed for robust domain reputation management.
Razor, specifically Vipul's Razor, is an older distributed spam detection and filtering system. Unlike many commercial email blacklists (or blocklists) used by major internet service providers (ISPs) and mailbox providers, Razor functions more as a collaborative, open-source project. It relies on user submissions of spam to build a database of spammy content and domains.
Its primary integration is often with spam filtering software like SpamAssassin. When an email or message is received, SpamAssassin can query the Razor database. If a match is found for the content or domain, a spam score is increased, potentially leading to the message being flagged or rejected. This means its impact is more localized and dependent on the specific spam filters employed by the receiving end, rather than a universal block by all major providers.
While a listing on Razor may not directly halt your email campaigns across the board, it indicates that your domain has been associated with content deemed unwanted by users, which can be a concerning signal. It's a reminder that even older, open-source blocklists serve as indicators of problematic sending behaviors.
The hidden link: SMS complaints and email domains
The critical, yet often overlooked, link between SMS spam complaints and email domain reputation lies in the domain itself. Imagine a scenario where a company sends out SMS messages, perhaps for debt collection, and these messages include a link or a mention of their primary domain, such as connect.example.com. If recipients find these SMS messages unwelcome and mark them as spam, even though they are SMS, the domain itself is associated with the negative feedback.
Spam reporting mechanisms are not always siloed by communication channel. Some systems, like Razor, are designed to identify and flag patterns of spam irrespective of the medium. If your domain is consistently present in SMS messages that are being reported, it can be added to the Razor blocklist. Even if your email activity from that domain is clean, the negative signals from the SMS channel contribute to its overall domain reputation.
Mailbox providers and spam filters look at a comprehensive set of signals to determine trustworthiness. While a Razor listing due to SMS complaints might not directly trigger email blocks by Gmail or Yahoo, it certainly contributes to a broader negative perception of your domain. This can make it harder for your emails to reach the inbox, as your domain's credibility is quietly eroded by activities outside the email ecosystem.
Broader implications for domain reputation
Domain reputation is a holistic measure that major mailbox providers use to assess the trustworthiness of a sending domain across all its uses. It's not just about what happens with your emails. Every interaction associated with your domain, whether via email, SMS, or even web activity, can contribute to its overall standing. When users mark a message (SMS or email) as spam, it generates a spam report or complaint signal that is fed into reputation algorithms.
Direct email spam complaints
Direct impact: These are directly reported to mailbox providers, leading to immediate reputation dips for the sending IP and domain.
Cause: Poor list hygiene, irrelevant content, high sending volume, or lack of consent for email subscriptions.
Mechanism: Users click the 'report spam' button in their email client.
Impact on email: Directly degrades sender reputation, leading to lower inbox placement and potential blocklisting (or blacklisting).
Remediation: Immediate list segmentation, re-engagement campaigns, and content review.
SMS spam complaints
Mechanism: Users block the number or report the SMS as spam via their mobile carrier.
Impact on email: Indirectly affects email domain reputation if the domain is present in the SMS message, leading to potential inclusion on shared blocklists like Razor.
Therefore, even if your SMS program is handled by a separate vendor or system, the domain names used within those messages are still tied to your overall brand reputation. A sudden spike in SMS complaints can trigger alarms in shared intelligence systems, which then flag your domain as potentially spammy. This can affect how your sender reputation is perceived by major email providers.
Strategies for reputation protection
The first step in addressing any blocklist (or blacklist) listing, including Razor, is understanding the cause. For Razor, it's often linked to content patterns. If the listing is indeed due to SMS complaints, the solution lies in optimizing your SMS sending practices rather than just your email. Ensure that your SMS contacts are explicitly opted-in and that the content is clear, relevant, and expected.
While Razor itself doesn't offer a direct delisting procedure due to its open-source nature, improving your sending behavior is key. This means reducing spam complaints across all channels. Regularly monitor your domain's health using tools that check against various blocklists and observe your email deliverability rates.
Furthermore, consider using distinct subdomains for different communication streams or purposes, especially if one stream is inherently more prone to complaints (e.g., debt collection). This isolation can help prevent issues from one subdomain from negatively impacting the reputation of your primary domain or other subdomains. Consistent monitoring and adherence to best practices for all digital communications are crucial for maintaining a strong and positive domain reputation.
Views from the trenches
Best practices
Maintain explicit consent for all recipients across email and SMS campaigns to reduce unwanted messages.
Segment your audience precisely for each communication channel, ensuring relevancy and expected content.
Regularly monitor your domain's health, including its presence on both email and content-based blacklists (blocklists).
Implement strong authentication for your email domain, such as SPF, DKIM, and DMARC, to prove legitimacy.
Use distinct subdomains for different communication types, especially for high-volume or sensitive messaging streams.
Common pitfalls
Assuming that SMS spam complaints have no impact on your email domain's reputation.
Failing to review the content of SMS messages for domain mentions that could trigger blocklist (blacklist) detections.
Relying solely on email validation for list hygiene without considering consent for SMS campaigns.
Ignoring older or less prominent blocklists like Razor, believing they have no significant deliverability impact.
Neglecting to align your email and SMS communication strategies, leading to inconsistent brand messaging.
Expert tips
Ensure your unsubscribe process is clear and accessible across all communication channels, fostering recipient trust.
Actively analyze complaint patterns across both email and SMS to identify and resolve root causes of dissatisfaction.
Consider a warming-up strategy for new domains or subdomains used in SMS, similar to email practices, to build trust.
Educate your team on the interconnectedness of domain reputation across all digital communication streams.
Leverage analytics from both email and SMS platforms to get a holistic view of user engagement and complaint rates.
Expert view
Expert from Email Geeks says: SMS spam complaints for a domain used in those messages can cause that domain to be flagged as spammy, even if the domain is not actively used for email sending.
2021-10-06 - Email Geeks
Marketer view
Marketer from Email Geeks says: A deliverable email address does not guarantee subscriber consent, as recipients might provide addresses that aren't their own, potentially leading to blocklist listings. However, older open-source projects like Razor tend to have minimal impact on overall deliverability.
2021-10-06 - Email Geeks
Navigating cross-channel reputation
The journey of an email from sender to inbox is complex, governed by a multitude of factors that contribute to your domain's reputation. While direct email practices are paramount, it's clear that signals from other digital communication channels, such as SMS, can indirectly influence this reputation.
A listing on a blocklist (or blacklist) like Razor, even if triggered by SMS spam complaints, is a signal that requires attention. It serves as an early warning that your domain is being associated with unwanted communications, which can eventually impact your email deliverability rates if left unaddressed. Proactive reputation management demands a holistic view of all your digital outreach.
By understanding these interconnected dynamics and implementing a comprehensive strategy that prioritizes consent, relevant content, and continuous monitoring across all channels, you can safeguard your email domain's reputation and ensure your messages consistently reach their intended audience.
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