Summary
ISPs like Optimum, Windstream, and CenturyLink utilize a mix of home-grown filtering, Spamhaus, Spamcop, internal blacklists, and SpamAssassin with custom rules. Content-related blocks are often triggered by specific words, problematic attachments, aggressive messaging, or poor sender reputation. Troubleshooting involves checking blacklists, analyzing content and headers for triggers, understanding SMTP error codes, authenticating email with SPF, DKIM, and DMARC, warming up new IPs and domains, managing sending volumes, and adhering to acceptable use policies.
Key findings
- Filtering Variety: ISPs employ a variety of filtering methods, including home-grown systems and third-party blacklists.
- Content Sensitivity: Content, including specific words, links, and attachments, significantly impacts filtering.
- Reputation Matters: Sender reputation is crucial; a poor reputation leads to blocks and deliverability issues.
- Authentication is Key: Proper email authentication (SPF, DKIM, DMARC) is essential for improving deliverability.
- Email Warmup Required: Email Warmup has become a requirement to ensure deliverability on new IP addresses and new sending domains.
Key considerations
- Blacklist Monitoring: Regularly check your IP against known blacklists and take steps to delist if necessary.
- Content Optimization: Use spam checker tools to identify and remove problematic content before sending.
- Authentication Implementation: Implement and properly configure SPF, DKIM, and DMARC to authenticate your email and establish trust.
- Sending Volume Awareness: Manage sending volumes to prevent blocks, especially when using new IPs or domains.
- Policy Compliance: Adhere to ISP acceptable use policies to avoid account penalties.
- Error Code Analysis: Pay attention to SMTP error codes for troubleshooting email delivery problems.
- Domain and IP Warmup: Warmup your domain and IP slowly when starting a new email sending process to ensure a smooth delivery.
What email marketers say
9 marketer opinions
ISPs like Optimum, Windstream, and CenturyLink employ a combination of methods to filter email, including Spamhaus, Spamcop, internal blacklists, and SpamAssassin with customized rules. Content-related blocks often result from trigger words, problematic attachments, or poor sender reputation. Troubleshooting involves checking blacklists, analyzing SMTP error codes, using spam checker tools, authenticating email with SPF, DKIM, and DMARC, warming up new IPs/domains, and ensuring consistent sending volumes.
Key opinions
- Filtering Methods: These ISPs use a mix of Spamhaus, Spamcop, internal blacklists, and SpamAssassin.
- Content Triggers: Blocks often stem from trigger words, executable attachments, or problematic phrases.
- Reputation Matters: Sender reputation is crucial; a poor reputation leads to blocks.
- Authentication: Proper email authentication (SPF, DKIM, DMARC) helps improve deliverability.
- Email Warmup: Email Warmup helps prevent blocks and improve deliverability on new IP addresses and new sending domains.
Key considerations
- Blacklist Checks: Regularly check your IP against known blacklists (Spamhaus, Spamcop).
- Content Analysis: Use spam checker tools to identify and remove problematic content before sending.
- Authentication Setup: Implement SPF, DKIM, and DMARC to authenticate your email and improve sender reputation. Properly setup SPF to define all allowed subnets to send email from.
- Email Warmup Scheduling: Gradually increase sending volumes when using new IPs or domains to establish a positive sending history.
- Monitor Sending Volumes: Monitor sending volumes to prevent blocks and improve deliverability.
Marketer view
Email marketer from Email Deliverability Blog shares that improving sender reputation is crucial to avoid blocks. She recommends authenticating your email with SPF, DKIM, and DMARC, and consistently sending valuable content to engaged subscribers.
15 Nov 2021 - Email Deliverability Blog
Marketer view
Email marketer from Reddit r/emailmarketing shares that smaller ISPs like Optimum, Windstream, and CenturyLink often use a combination of Spamhaus, Spamcop, and internal blacklists. They recommend checking your IP against these blacklists first.
5 Oct 2023 - Reddit r/emailmarketing
What the experts say
6 expert opinions
ISPs like Optimum, Windstream, and CenturyLink may rely on home-grown filtering systems. Content, links, messaging style, and sender reputation are major factors in email deliverability. Understanding SMTP error codes aids in pinpointing the reason for blocks. Analyzing message content and headers to identify triggers is crucial for troubleshooting. Maintaining a positive sender reputation by building your brand, staying relevant, and being trustworthy is also essential.
Key opinions
- Filtering Systems: ISPs might use home-grown filtering systems, possibly all similar due to acquisitions.
- Content Matters: Content, links, and messaging style influence filtering; 'aggressive' messaging can be problematic.
- SMTP Codes: Understanding SMTP error codes helps diagnose delivery issues.
- Reputation Impact: IP address and sender reputation are vital for deliverability and avoiding blocks.
Key considerations
- Content Analysis: Analyze email content and headers to identify potential triggers for spam filters.
- Reputation Management: Actively manage and improve your sender reputation through brand building, relevance, and trustworthiness.
- Testing: Use spam filter simulators to test emails before sending.
- Error Code Review: Pay attention to SMTP error codes for troubleshooting email delivery problems.
Expert view
Expert from Spam Resource explains that understanding SMTP error codes is important for troubleshooting email delivery issues. He highlights that 5xx errors are permanent failures and 4xx errors are temporary failures. The specific code helps pinpoint the reason for the block, such as content filtering or reputation issues.
12 Jan 2024 - Spam Resource
Expert view
Expert from Email Geeks explains that these ISPs mostly use home-grown filtering. She notes that Qwest and Centurylink are the same, and they bought part of Embarq, suggesting they might all be the same now.
21 Sep 2023 - Email Geeks
What the documentation says
4 technical articles
Spamhaus utilizes IP and domain-based blacklists to combat spam. CenturyLink prohibits spam, phishing, and malicious content under its Acceptable Use Policy, with violations leading to account consequences. DKIM is an email authentication method that uses digital signatures to verify sender identity and enhance deliverability. DMARC builds upon SPF and DKIM by providing a policy for handling emails failing authentication checks, instructing recipient servers on how to manage unauthenticated messages.
Key findings
- Spamhaus Blacklists: Spamhaus employs IP and domain-based blacklists to identify and block spam sources.
- CenturyLink Policy: CenturyLink prohibits spamming, phishing, and the transmission of malicious content, leading to account suspension if violated.
- DKIM Authentication: DKIM uses digital signatures to verify sender identity, preventing spoofing and improving deliverability.
- DMARC Policy: DMARC provides a policy based on SPF and DKIM for handling unauthenticated emails, guiding recipient servers on how to process them.
Key considerations
- Blacklist Awareness: Understand Spamhaus's criteria for blacklisting and the delisting process.
- Policy Compliance: Adhere to CenturyLink's Acceptable Use Policy to avoid account penalties.
- DKIM Implementation: Implement DKIM to authenticate your emails and prevent spoofing.
- DMARC Configuration: Configure DMARC to establish a policy for handling unauthenticated emails and protect your domain.
Technical article
Documentation from DKIM.org details that DKIM (DomainKeys Identified Mail) is an email authentication method that uses a digital signature to verify the sender's identity. It helps prevent spoofing and improves deliverability.
10 Dec 2021 - DKIM.org
Technical article
Documentation from DMARC.org details that DMARC (Domain-based Message Authentication, Reporting & Conformance) is an email authentication protocol that builds upon SPF and DKIM to provide a policy for handling emails that fail authentication checks. It allows domain owners to specify how recipient mail servers should handle unauthenticated messages.
8 Feb 2025 - DMARC.org
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