Can different IP addresses or sub-domains affect deliverability differently for the same sender?

Absolutely. Your sender reputation is not static and can vary across different sending IPs or sub-domains you use. While your main domain might have a strong reputation, a specific IP or sub-domain used for a particular campaign might have a lower standing, leading to inconsistent deliverability. Additionally, different types of emails (e.g., transactional vs. marketing) can build different reputations, even under the same primary domain.

How can the content of an email from the same sender cause some to go to spam and others to inbox?

Content plays a significant role. Even minor variations in subject lines, body text, image-to-text ratio, or the presence of certain keywords can trigger spam filters. If one email contains elements that seem suspicious to a filter, it might be flagged, whereas another, cleaner email from the same sender will pass through. Engagement with specific content also influences its deliverability.

Do individual recipient actions like marking as not spam affect future emails from that sender?

Yes, recipient actions are powerful. If a recipient frequently moves your emails from spam to their inbox or adds your address to their contacts, their personal filter will learn to trust you, improving deliverability for future emails to that specific recipient. Conversely, if they mark your emails as spam, even legitimate ones, their filter will learn to send those messages to spam.

How important are technical settings like SPF, DKIM, and DMARC for consistent deliverability?

Ensuring proper SPF, DKIM, and DMARC configuration is critical for consistent deliverability. Even slight misconfigurations or temporary issues with DNS lookups can lead to authentication failures for some emails, causing them to be flagged as suspicious. Consistent authentication across all sends builds trust with mailbox providers.

Why do some emails from the same sender go to spam while others go to inbox? - Sender reputation - Email deliverability - Knowledge base

Q: Why do some emails from the same sender go to spam while others go to the inbox?

Mailbox providers use complex algorithms that evaluate each email individually. Factors like the specific sending IP, sub-domain reputation, email content variations, recipient engagement with that particular email, and even dynamic changes in spam filters can cause this inconsistency. One email might tick all the right boxes, while another, even from the same sender, might trigger a filter due to a minor change or a recipient's specific past behavior.

It can be incredibly frustrating to see some of your emails land perfectly in the inbox, while others from the exact same sender (or even the same campaign) end up in the spam folder. This inconsistency often leaves senders scratching their heads, wondering what went wrong with specific messages.

The truth is, email deliverability is a complex dance between many factors. Mailbox providers (like

Gmail and

Outlook) employ sophisticated algorithms that evaluate every single email based on a wide array of signals. These signals can vary from message to message, even if the sender remains constant.

Understanding these nuances is key to improving your overall inbox placement. We'll explore the primary reasons behind this common deliverability puzzle, delving into aspects like sender reputation, content quality, technical setup, and recipient behavior that can cause such seemingly erratic outcomes.

Understanding sender reputation

Sender reputation is arguably the most critical factor in email deliverability, and it's not a monolithic score. It's built on a combination of your IP address reputation, your sending domain's reputation, and even sub-domain reputations. If you're sending from a pool of IP addresses or using various sub-domains (e.g., for transactional versus marketing emails), the reputation of each specific sending entity can differ. A single poorly performing IP or sub-domain could drag down the deliverability of emails sent through it, even if other streams are performing well.

The consistency of your sending volume and frequency also plays a significant role. Sudden spikes in sending, or sending to a new, unengaged segment, can negatively impact your reputation for those specific sends. This is particularly true for emails from new domains with low sending volumes. Mailbox providers look for predictable sending patterns. Deviations can raise red flags, causing some emails to be filtered.

Furthermore, your sender reputation is constantly being evaluated. While your overall domain reputation might be good, if a specific campaign or batch of emails triggers a high number of spam complaints, that particular send's reputation can suffer. This localized impact can result in some emails bypassing filters while others get caught in them. Being listed on an email blocklist (or blacklist) is another major factor that can cause inconsistent deliverability. If your IP or domain appears on a blocklist, even for a short period, some emails may be blocked outright or sent to spam, while others might still sneak through to inboxes, depending on the recipient's specific mail server configurations and the strictness of their spam filters.

For example, if you have a history of good sending, but suddenly a small segment of your list, perhaps an old, unengaged one, starts marking your emails as spam, it can temporarily lower the reputation associated with those specific sends. This doesn't necessarily tank your entire sending domain's reputation, but it can create the observed inconsistency where some emails arrive while others don't.

Positive reputation signals

High engagement: Consistent opens, clicks, and replies from recipients.
Low complaint rates: Few recipients marking your emails as spam.
Valid recipients: Minimal bounces and very few, if any, spam trap hits.

Negative reputation signals

High complaint rates: Users frequently marking your emails as spam.
Spam trap hits: Sending to invalid or old email addresses.
Blocklist listings: Your IP or domain appearing on a public or private blocklist (blacklist).

Content and engagement dynamics

The content of your email can also influence whether it lands in the inbox or spam folder. Spam filters analyze various elements within your message, including keywords, image-to-text ratio, link quality, and even HTML structure. If some emails contain common spam trigger words, excessive images, broken links, or suspicious formatting, they might be flagged, even if other emails from the same sender are clean.

Engagement is another major determinant. Mailbox providers track how recipients interact with your emails. If a particular email receives high engagement (opens, clicks, replies) from its recipients, it signals to the ISP that the content is desired. Conversely, if another email, even from the same sender, sees low engagement, high deletion rates, or is marked as spam by many recipients, its deliverability can suffer. This is why some emails can go to spam even with an otherwise low spam rate.

Maintaining a clean and engaged email list is crucial. Sending emails to inactive or unengaged subscribers can dilute your engagement metrics, signaling to ISPs that your content is not valued, potentially leading to increased spam placements. This issue is often seen when emails go to spam because of poor list management.

Email content and engagement best practices

Personalization: Tailor content to recipient interests to boost engagement.
Clear CTAs: Make it easy for recipients to interact with your email.
List hygiene: Regularly remove unengaged subscribers and invalid addresses.
A/B testing: Test subject lines and content to see what resonates best.

Technical configurations and their impact

Even with a strong reputation and good content, technical misconfigurations can cause inconsistent deliverability. Email authentication protocols such as SPF, DKIM, and DMARC are crucial. If these records are incorrectly set up, or if there are intermittent issues with their validation, some emails might pass authentication while others fail. For example, a sender might have a valid SPF record but if the sending IP changes frequently or is not always covered by the SPF policy, some emails may fail SPF checks.

DMARC alignment is particularly important. While an email might pass SPF and DKIM individually, if the domains in the "From" header (RFC 5322.From) do not align with the SPF or DKIM authenticated domains, DMARC will fail. This can lead to emails landing in spam, as highlighted by email authentication guides. Small changes in email headers, sending platforms, or even the inclusion of third-party tracking pixels can affect how these authentication checks perform on a per-email basis.

Example of Authentication-Results Headeremail-header

Authentication-Results: mx.google.com;
       dkim=pass header.i=@act.democrats.org header.s=s1 header.b=dUqYiWoJ;
       dkim=pass header.i=@sendgrid.info header.s=smtpapi header.b=lTk1IKMt;
       spf=pass (google.com: domain of bounces+1584542-ad7b-wttwdta1=gmail.com@sg.act.democrats.org designates 149.72.146.119 as permitted sender) smtp.mailfrom="bounces+1584542-ad7b-wttwdta1=gmail.com@sg.act.democrats.org";
       dmarc=pass (p=QUARANTINE sp=QUARANTINE dis=NONE) header.from=act.democrats.org

The impact of DMARC policies

A DMARC policy tells receiving servers what to do if an email fails authentication checks. Even if your policy is set to 'quarantine' or 'none', inconsistencies in SPF or DKIM alignment can still cause some emails to land in spam while others are delivered. Implement DMARC monitoring to catch these issues. Read our simple guide to DMARC, SPF, and DKIM for more.

Recipient-specific factors

Beyond sender-controlled factors, individual recipient behavior and mailbox provider settings heavily influence where an email lands. A user might manually move an email from the spam folder to their inbox, effectively whitelisting future emails from that sender. Conversely, if a user consistently marks emails as spam, even legitimate ones, their personal filter will learn to classify similar messages as junk. This explains why some emails can go to spam while others go to inbox from the same sender with Gmail.

Many users also set up custom inbox rules or filters that can inadvertently send legitimate emails to spam. These rules can be based on subject lines, keywords, or even specific sender addresses. If a new email happens to trigger one of these individual rules, it might bypass the inbox despite the sender's good standing.

Mailbox providers also use adaptive filtering based on aggregate user behavior. If a large segment of recipients from a particular ISP (Internet Service Provider) consistently engages positively with your emails, it builds a stronger reputation for your sending practices with that specific provider. Conversely, if a new campaign or message type garners negative feedback (e.g., spam complaints, low opens) from users on a specific ISP, subsequent similar emails might be directed to spam by that ISP's filters, leading to the observed inconsistency across different emails from the same source.

Recipient action	Impact on deliverability
Moving to inbox	Strong positive signal, improves sender reputation for that recipient.
Marking as spam	Strong negative signal, significantly hurts sender reputation.
Deleting without opening	Negative signal, indicates disinterest or irrelevant content.
Adding to contacts	Very strong positive signal, almost guarantees future inbox placement.
Replying	Exceptional positive signal, shows high engagement and relevance.

Views from the trenches

Best practices

Maintain consistent sending volumes and frequencies for different email streams.

Segment your audience effectively and tailor content to each segment's interests.

Regularly clean your email lists to remove inactive or unengaged subscribers.

Implement and monitor DMARC, SPF, and DKIM for all sending domains.

Educate your subscribers on adding your email to their safe sender list.

Common pitfalls

Using the same 'From' address for vastly different content types (e.g., transactional vs. marketing).

Ignoring spam complaints or low engagement metrics on certain campaigns or segments.

Not aligning all email authentication protocols (SPF, DKIM, DMARC) consistently.

Failing to monitor sender reputation across all sending IPs and sub-domains.

Sending to purchased lists or unverified recipients, leading to spam trap hits.

Expert tips

Use Google Postmaster Tools to analyze your deliverability data for Gmail recipients.

Set up feedback loops (FBLs) with major ISPs to identify and address spam complaints promptly.

Employ a dedicated IP for high-volume or critical transactional emails to isolate their reputation.

Consider using different sub-domains for distinct email categories (e.g., newsletter.yourdomain.com, transactional.yourdomain.com).

Run small, targeted tests on new content or segments before a full send to gauge initial reaction.

Marketer view

Marketer from Email Geeks says they noticed a pattern where the first one or two messages a recipient receives often have very low open rates, which then jumps significantly for subsequent messages, indicating a reputation-based filtering.

2022-10-07 - Email Geeks

Expert view

Expert from Email Geeks says that they tried signing up for a political campaign's emails and observed that initial messages went to spam, but later emails from the same sender landed in the inbox, even without manual interaction.

2022-10-07 - Email Geeks

Ensuring consistent inbox placement

The phenomenon of some emails going to spam while others reach the inbox, even from the same sender, is a clear indication that deliverability is a dynamic and granular process. It's rarely a single issue but rather a combination of variable factors related to sender reputation, email content, technical configurations, and individual recipient behavior. Proactive monitoring of your sending performance across all streams, meticulous list management, and ensuring robust email authentication are essential for achieving consistent inbox placement.