What is the best IP network configuration and email volume strategy for email service providers to avoid deliverability issues?

Key findings

• IP network configuration: A single, larger IP block, such as a /22, is generally more advantageous for deliverability than multiple smaller, scattered blocks (e.g., /27s). Larger blocks reduce maintenance overhead and avoid triggering 'snowshoe spammer' flags at ISPs.
• Avoiding spammer perception: Scattered IP addresses can make an ESP’s traffic resemble that of snowshoe spammers or those cycling through disposable virtual private servers (VPSes).
• Volume per IP: For significant daily volumes (e.g., 500,000 emails to a major ISP like Yahoo/AOL), a single IP or a very small number (1-2) is often sufficient and preferred over a large pool of IPs. Excessive IPs for a given volume can be counterproductive, hinting at snowshoeing.
• Root cause of deferrals: Temporary deferrals and blocklist messages (e.g., TS04 errors from Microsoft) often stem from high complaint rates due to unwanted email content, irrelevant links, or poor sending domains, rather than purely technical IP configuration issues or low volume.
• IP warming: Gradually increasing email volume from new IPs helps establish credibility and build a positive sending reputation with Internet Service Providers (ISPs). This process is vital to avoid immediate deliverability issues.

Key considerations

• Quality over quantity: The primary focus should be on the quality of mail being sent. If users are complaining or marking emails as spam, adding more IPs or changing network configuration will not resolve the underlying deliverability problems.
• List hygiene: Regularly cleaning and verifying contact lists, removing inactive subscribers and spam traps, is fundamental. Sending to disengaged or unknown recipients damages sender reputation and leads to complaints.
• Microsoft blocklists: When facing blocklists from Microsoft, direct communication and opening a support ticket are often necessary steps for unblocking IPs, as the messages can be generic and not specific to IP configuration.
• Dedicated IP usage: For senders consistently exceeding 1,000,000 emails per month, dedicated IPs are recommended. These offer greater control over your email-sending health and the ability to build a specific IP reputation, though they also entail full responsibility for that reputation. Smaller senders may find shared IPs more effective. Learn more about dedicated IP benefits.
• IP warming strategies: A robust IP warming strategy is essential when introducing new IP addresses or scaling volume to avoid deliverability degradation. This involves gradually increasing the sending volume over time to build trust with ISPs.

Key opinions

• IP configuration dilemmas: Marketers frequently question whether to use IPs on different networks or contiguous blocks, aiming to optimize for deliverability rather than just ease of acquisition.
• Microsoft blocklist challenges: Encountering generic Microsoft blocklist messages, especially for previously unused IPs, is a common concern. This leads to questions about whether to acquire new IP batches or focus on unblocking existing ones.
• Volume scaling uncertainty: There's often uncertainty about the ideal number of IPs for a given daily email volume, with marketers sometimes overestimating the number needed (e.g., 10-50 IPs for 500k emails) and fearing under-provisioning.
• Low volume deferrals: Even with low sending volumes, marketers observe frequent message deferrals or 'backoff mode' from Mail Transfer Agents (MTAs), leading them to suspect complaint rates per IP as the cause.

Key considerations

• Content relevance: Crafting useful and relevant emails that meet audience needs is paramount to avoid spam complaints and improve open rates.
• List verification: Routinely verifying contact lists and removing inactive or unsubscribed contacts, along with spam traps, is a basic but critical practice for improving deliverability.
• Engagement impact: Sending high volumes to unengaged contacts or those who frequently mark emails as spam will negatively impact deliverability, regardless of IP setup. Maintaining a clean list and focusing on engaged subscribers is more important than IP quantity.
• Building IP reputation: A dedicated IP provides more control over email sending health, allowing marketers to build their own reputation, especially for transactional emails.

Key opinions

• Spam avoidance is key: Any IP configuration can work effectively as long as the sender avoids spamming. Issues primarily arise when nearby IP ranges (even those not belonging to the ESP) are engaged in spamming.
• Larger IP blocks are better: A single /22 IP block is significantly better for deliverability than multiple smaller blocks like 32 /27s. It is also marginally better than four /24s, mainly due to reduced maintenance complexity which lowers the chance of errors.
• Snowshoe spamming detection: Using multiple scattered smaller IP ranges, such as /27s, can cause traffic to be flagged as 'snowshoe spamming' by ISPs, even before content analysis.
• Appropriate IP volume: For daily volumes around 500,000 emails to major providers like Yahoo/AOL, one or two IPs are generally sufficient. Using 10-50 IPs for this volume is considered excessive and can trigger snowshoe spamming alerts.
• Complaints are the real issue: If MTAs are deferring messages with 'unexpected volume or user complaints' errors, the problem is not aggressive filters but rather that recipients do not want the mail. This means the content, links, or sending domains are generating complaints.
• No technical fix for unwanted mail: When ISPs put IPs into 'backoff mode' due to unwanted mail, there is no technical configuration fix. The only solution is to stop sending mail that generates complaints.

Key considerations

• Addressing Microsoft blocklists: For Microsoft blocklist issues, the standard procedure is to open a support ticket and present your case directly to them. This is often the most effective way to resolve generic blocklist notifications.
• Don't snowshoe: Attempts to 'snowshoe' (distribute volume across many IPs) will not accelerate mail acceptance at major ISPs like Yahoo, as they evaluate the entire message content and sender reputation. This can lead to further blocklists or deferrals.
• Tuning backoff modes: While the fundamental fix for deferrals is to stop sending unwanted mail, Message Transfer Agent (MTA) backoff modes can sometimes be overly aggressive. Adjusting settings to not trigger on the first few TS04 errors might be considered as a temporary measure, although this cannot be set in PMTA (PowerMTA) directly.
• Proactive monitoring: Being listed on a major blacklist can severely impact deliverability. Proactive monitoring and quick action to delist are essential for maintaining sender reputation and inbox placement.
• Consistent sending habits: IP reputation is tied to sending habits. Consistent, legitimate sending builds positive reputation, while sudden spikes or sending spam can quickly damage it, leading to deliverability degradation.

Key findings

• Dedicated IP recommendation: Documentation generally recommends dedicated IPs for senders consistently exceeding a high volume of emails per month, such as 1,000,000, to gain more control over their sending reputation.
• Shared IP suitability: For smaller senders or those with less consistent volume, shared IPs are often more effective, as the reputation responsibility is distributed among multiple users.
• Gradual volume increase: Warming up an IP or domain involves gradually increasing the email volume over time. This process is crucial for establishing credibility with ISPs and preventing abrupt deliverability drops.
• Authentication protocols: Robust authentication protocols (like DMARC, SPF, and DKIM) are highlighted as essential features to look for in an ESP, as they help monitor and improve sender reputation.

Key considerations

• Consistency in sending: ISPs value consistent sending behavior. Sudden, large spikes in email volume from a new or cold IP can be perceived as suspicious and lead to deliverability issues.
• Domain warming: Beyond just IP warming, gradually shifting traffic from new domains in email headers is also a best practice, starting with small percentages and increasing over time.
• Content and engagement: While not directly an IP or volume strategy, documentation consistently links content quality, audience engagement, and list hygiene as critical factors that, if overlooked, will undermine any IP or volume strategy.
• Monitoring and analytics: Comprehensive analytics and monitoring tools are advised to track email deliverability metrics and identify issues early, allowing for timely adjustments to sending practices. Learn more about avoiding spam folders.