How do mailbox providers define and identify bulk email senders?

Key findings

• Volume isn't the sole factor: Mailbox providers consider senders "bulk" based on consistent high volume, regardless of whether a fixed numerical threshold (e.g., 5,000 messages per day) is met.
• Sending patterns matter: Distributing messages across many different domains or IP addresses to circumvent volume limits will still likely result in being identified as a bulk sender.
• Authentication is critical: All bulk senders must adhere to email authentication standards such as SPF and DKIM. DMARC implementation is also becoming a strict requirement for higher volume senders.
• Reputation is paramount: IP and domain reputation play a significant role. Even authenticated mail can be blocked if engagement is low or spam complaints are high.

Key considerations

• Don't game the system: Attempting to bypass bulk sender rules by splitting volume across multiple domains or subdomains is generally ineffective and can lead to worse deliverability.
• Invest in authentication: Ensure your sending domains are properly authenticated with SPF, DKIM, and DMARC. Understanding these protocols is vital for modern email deliverability (check our simple guide to DMARC, SPF, and DKIM). Read more about Gmail's official sender guidelines for more.
• Monitor sender reputation: Proactively track your domain and IP reputation using tools like Google Postmaster Tools. This helps identify issues before they severely impact delivery (see how domain reputation is measured).
• Focus on engagement: Prioritize sending relevant emails to engaged subscribers. Low engagement or high spam complaints are strong indicators of poor sender quality, regardless of volume.

Key opinions

• Threshold misunderstanding: Many marketers initially interpreted the 5,000 message rule as a strict cap, believing they could avoid bulk sender requirements by staying just below it.
• Authentication importance: There's a growing awareness that authentication (SPF, DKIM, DMARC) is non-negotiable for all serious senders, regardless of perceived "bulk" status.
• Domain proliferation challenges: Some marketers considered spinning up many new domains to stay under perceived volume limits, but this is recognized as an unsustainable and ultimately ineffective strategy.
• Cold email difficulties: Marketers engaging in cold B2B email are finding it increasingly difficult to achieve inbox placement due to these stricter rules.

Key considerations

• Educate clients: It's crucial for marketing agencies and deliverability consultants to educate their clients about the actual complexities of bulk sender definitions beyond simple volume numbers (see recent changes to Google's bulk sender guidelines).
• Strategic sending: Marketers should move away from "trashy tactics" and focus on genuine, permission-based marketing to build positive sender reputation. Learn more about new rules for bulk email senders.
• Subdomain strategy: While subdomains can help segment traffic, relying on them to fully isolate bad sending practices from the main domain is not a foolproof solution against sophisticated mailbox provider detection.
• Unsubscribe headers for all marketing mail: Even for seemingly "personal" sales emails, if sent at scale, including a List-Unsubscribe header is becoming a de facto best practice (learn how to comply with Gmail's rules).

Key opinions

• 5k is a guideline: Experts clarify that Google's 5,000 message threshold is not a hard limit, and senders exceeding it are not automatically blocked, nor are those below it guaranteed delivery.
• Intent matters: Mailbox providers aim to distinguish legitimate bulk senders from spammers who try to disguise their volume or tactics.
• Identification capabilities: Receivers have advanced capabilities to identify a single bulk sender even if they use multiple domains or IP addresses.
• P2P vs. bulk distinction: True person-to-person (P2P) emails are generally exempt from bulk sender guidelines, but disguised marketing or sales emails will be treated as bulk.

Key considerations

• Embrace authentication: Experts strongly recommend that all senders, especially those with significant volume, adopt strong authentication (SPF, DKIM, DMARC) as a foundational requirement. This will verify sending domains and IP addresses as legitimate. For more details, see our article on new bulk sender requirements.
• Reputation is cumulative: The reputation of a sender's aligned DKIM domain is key, but the reputation of the ESP's shared infrastructure (ESP DKIM domain) also plays a role. A negative reputation for shared domains can lead to blacklisting (or blocklisting) for everyone using them (check our guide to how email blacklists actually work).
• Don't try to evade: Attempting to "snowshoe" (spread volume across many domains/IPs) or disguise bulk mail as personal emails will be detected and negatively impact deliverability.
• Maintain consistent good behavior: Focus on consistent good sending practices across all identities and platforms to build and maintain a positive sender reputation (learn about how ISPs track engagement).

Key findings

• Thresholds are stated: Google explicitly defines bulk senders as those sending 5,000 or more messages per day to Gmail accounts, requiring specific compliance measures.
• Authentication is mandatory: For bulk senders, authenticating outgoing email with SPF, DKIM, and increasingly DMARC is a non-negotiable requirement.
• Low spam rates: Senders must maintain a very low spam complaint rate, often below 0.1%, to ensure good deliverability.
• Easy unsubscribes: A one-click unsubscribe mechanism must be provided for recipients to easily opt-out, especially for marketing and promotional emails.
• Domain alignment: DMARC policy applies to the "From" header, requiring proper alignment of SPF and DKIM with the sending domain.

Key considerations

• February 2024 deadlines: Major changes (e.g., DMARC enforcement, one-click unsubscribe) came into effect in February 2024, impacting bulk senders across Gmail and Yahoo (see our guide on recent changes to Google's bulk sender guidelines and an overview of bulk sender changes at Yahoo/Gmail).
• Content and reputation: Beyond technical compliance, documentation stresses the importance of sending desired content and maintaining a positive sender reputation to avoid blocklists.
• Monitoring tools: Mailbox providers offer tools (e.g., Google Postmaster Tools) for senders to monitor their performance and identify areas for improvement (learn more about the ultimate guide to Google Postmaster Tools V2).
• Holistic approach: Compliance is not just about meeting one requirement; it's about a holistic approach to email hygiene, engagement, and authentication.