How serious are MXToolBox blacklistings and what should I do about rented email lists?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 Jul 2025
Updated 19 Aug 2025
7 min read
When you use a tool like MXToolBox to check your domain or IP address, it often returns a list of blacklists (or blocklists) your sending infrastructure might appear on. It can be concerning to see multiple listings, and a common question arises: how serious are these particular blacklistings? This is especially pertinent when considering practices like using rented email lists, which often contribute to such listings.
The impact of a blacklist listing varies greatly depending on which list it is. Some blacklists are widely used by major internet service providers (ISPs) like Gmail and Outlook, while others have very limited reach. Understanding this distinction is crucial for assessing potential damage to your email domain reputation and deliverability.
When you check your domain or IP on a blacklist checker, you might see several listings. The key is to differentiate between those that genuinely impact your email deliverability and those that are less significant. For example, some blocklists, like SORBS.net, can have a moderate impact, particularly if you're sending to certain regions or smaller ISPs. Other less common lists (also called DNSBLs or RBLs) might be used by a minimal number of mail servers.
The majority of blacklists that MXToolBox flags are often not widely adopted by major mailbox providers. This means that while a listing might appear, it's unlikely to cause a significant uptick in your bounce rates or prevent your emails from reaching the inbox at large providers. The real concern isn't just being listed, but if that listing translates into actual delivery failures or widespread spam folder placement for your recipients.
However, it's worth noting that even minor blacklist inclusions can sometimes be an early indicator of underlying problems with your sending practices. While not immediately detrimental, they can signal a need to review your email acquisition process or sending patterns. It's about proactive email deliverability management rather than just reactive de-listing.
Blacklist Category
Example
Typical Impact
What to monitor
High Impact
Spamhaus, SpamCop
Immediate and widespread email blocking by major ISPs.
Significant bounce rates across many domains.
Moderate Impact
SORBS, Abusix
Potential blocking by some smaller ISPs or those with stricter policies.
Noticeable bounces from specific recipient domains.
Low Impact
Many MXToolBox-listed lists
Minimal to no noticeable impact on overall deliverability.
Isolated bounce reports, not impacting major email providers.
The perils of rented email lists
Rented, purchased, or shared email lists, regardless of how they are acquired or the stated consent within an association, are fundamentally problematic for email deliverability. The core issue is that the recipients on these lists have not directly and explicitly opted in to receive emails from *you*. This lack of direct consent means that your emails are, by definition, unsolicited.
Sending to rented lists significantly increases your risk of hitting spam traps and generating high complaint rates. Recipients who didn't sign up for your mail are far more likely to mark it as spam, even if they're members of an association that theoretically allows data sharing. This negative feedback is a strong signal to ISPs that your emails are unwanted.
Furthermore, these lists often contain outdated or invalid email addresses, leading to high bounce rates, particularly 550 bounce codes for bad-mailbox or bad-domain errors. A high volume of hard bounces indicates poor list hygiene to ISPs, which can severely damage your sender reputation and lead to your IP or domain being added to more impactful blacklists, or even IP getting blocklisted at the ISP level directly. Most reputable Email Service Providers (ESPs) also strictly prohibit the use of purchased or rented lists in their terms of service.
The risks of rented lists
Using rented email lists, even if your organization has some form of agreement for sharing contacts, is widely considered a spamming activity by mailbox providers. This practice leads to negative consequences that erode your email program's effectiveness over time.
Low Engagement: Recipients have not explicitly opted in, resulting in poor open and click rates and high spam complaint rates.
Reputation Damage: ISPs track complaints and low engagement, which directly harms your sender reputation and domain trust.
Blacklist Inclusions: Increased likelihood of being listed on critical blacklists, leading to widespread delivery issues.
High Bounce Rates: Rented lists contain many invalid or outdated addresses, leading to hard bounces that harm your sending reputation.
The long-term health of your email program relies on maintaining a strong sender reputation. This means consistently sending emails that are wanted and expected by your recipients. Any practice that generates high complaint rates, spam trap hits, or hard bounces will erode this reputation, regardless of initial blacklist status.
Major mailbox providers use sophisticated algorithms that go beyond simple blacklist checks. They analyze engagement metrics (opens, clicks), complaint rates, and whether your emails are consistently marked as spam. If your emails from Outlook and Gmail are consistently landing in the spam folder, that's a much more serious problem than a listing on a minor blacklist. This indicates an underlying issue with your email sending practices.
Building a positive sender reputation is a continuous process that involves adhering to best practices like obtaining explicit consent, segmenting your lists, and maintaining good list hygiene. This proactive approach helps you avoid the causes of blacklisting, rather than just reacting to them.
Legitimate list building
Direct Opt-in: Requires explicit consent from each recipient via signup forms on your website.
Clear Expectations: Recipients know what content they will receive and how often.
High Engagement: Leads to better open rates, clicks, and fewer complaints.
Positive Reputation: Builds trust with ISPs and ensures high inbox placement.
Rented list dangers
Indirect Consent: Recipients didn't sign up to hear from your specific organization.
Unwanted Mail: Perceived as spam, leading to high complaint rates and low engagement.
Bad Data: Often contain outdated addresses, spam traps, and honeypots, inflating bounces.
Reputation Ruin: Damages sender reputation, leading to hard blocks and deliverability collapse.
Actionable steps for improving deliverability
If you discover your domain or IP is on a blacklist, the first step is to assess its severity using a blocklist monitoring tool. For minor listings, focus on the underlying cause. If you're using rented lists, this is almost certainly the root of the problem, and addressing it is paramount.
For serious blocklistings (e.g., Spamhaus), you'll need to identify and rectify the cause of the listing. This typically involves stopping all problematic sending practices, cleaning your email list of invalid addresses and spam traps, and then submitting a delisting request to the blacklist operator. Each blacklist has its own delisting process.
The most effective long-term solution involves shifting away from any practices that lead to unsolicited mail. This means transitioning to permission-based email marketing, where every subscriber has explicitly agreed to receive your communications. This is the only sustainable path to excellent email deliverability and avoids the constant struggle with blocklists.
Prioritize Major Lists: Focus on delisting from highly impactful blacklists if affected.
Cease Rented Lists: Immediately stop sending to any purchased, rented, or shared email lists.
Implement Opt-in: Shift to an explicit, direct opt-in process for all new subscribers.
Clean Your List: Regularly remove inactive subscribers, hard bounces, and known spam traps.
Monitor Reputation: Continuously monitor your sender reputation and deliverability metrics.
Views from the trenches
Best practices
Always obtain direct and explicit consent from subscribers to avoid deliverability issues.
Regularly clean your email lists to remove inactive users and invalid addresses.
Monitor your sender reputation proactively to catch potential problems early.
Focus on content quality and segmentation to improve subscriber engagement.
Adhere strictly to your ESP's terms of service regarding list acquisition.
Common pitfalls
Relying on rented or purchased email lists for lead generation.
Ignoring minor blacklist listings, as they can be early warning signs.
Not removing hard bounces from your email list promptly.
Failing to track spam complaints and engagement metrics.
Assuming implied consent is sufficient for email marketing.
Expert tips
If your business model relies heavily on cold outreach, consider diversifying your communication channels instead of email.
Educate your team on the importance of permission-based email practices and long-term reputation.
Sometimes, a minor blacklist can be a
smoke indicative of someone walking around with a lit match.
It is a warning sign, not necessarily a catastrophe.
Marketer view
A marketer from Email Geeks says that many of the blacklists reported by MXToolBox are not a major problem.
2020-02-26 - Email Geeks
Expert view
An expert from Email Geeks says that SORBS may have a small impact, but the primary concern should be actual bounces.
2020-02-26 - Email Geeks
The path to better email deliverability
While MXToolBox blacklistings can seem daunting, their severity largely depends on the specific list. Many are minor and won't significantly impact your overall deliverability. However, any listing, especially when coupled with high bounce rates or spam complaints, should prompt a deeper investigation into your sending practices.
The use of rented email lists is a clear and consistent threat to email deliverability and should be avoided at all costs. It's a practice that inevitably leads to reputation damage, blacklistings (or blocklistings), and poor inbox placement. The path to reliable email delivery is paved with explicit consent and rigorous list hygiene. By focusing on these fundamentals, you can build a sustainable and effective email program that bypasses blacklists and consistently reaches your audience's inboxes.
Spamhaus
0Spam
Cisco
NoSolicitado
URIBL
abuse.ro
ALPHANET
Anonmails
Ascams
BLOCKEDSERVERS
Calivent Networks
EFnet
JustSpam
Kempt.net
NordSpam
RV-SOFT Technology
Scientific Spam
Spamikaze
SpamRATS
SPFBL
Suomispam
System 5 Hosting
Team Cymru
Validity
www.blocklist.de Fail2Ban-Reporting Service
ZapBL
2stepback.dk
Fayntic Services
ORB UK
technoirc.org
TechTheft
SORBS.net, can have a moderate impact, particularly if you're sending to certain regions or smaller ISPs. Other less common lists (also called DNSBLs or RBLs) might be used by a minimal number of mail servers.
Spamhaus, 
SpamCop
Abusix
lists
Email Service Providers (ESPs) also strictly prohibit the use of purchased or rented lists in their terms of service.
Outlook and 
Gmail are consistently landing in the spam folder, that's a much more serious problem than a listing on a minor blacklist. This indicates an underlying issue with your email sending practices.
