Why does SendGrid show 'Delivered' before a 'Block' for mailbox full?

SendGrid's 'Delivered' status means the recipient server initially accepted the email with a 250 OK response. A 'Block' for mailbox full occurs later, as an asynchronous notification, when the recipient server discovers it cannot store the email due to quota limits after initial acceptance.

What's the difference between a soft bounce (block) and a hard bounce in SendGrid?

A hard bounce is an immediate, permanent rejection (e.g., recipient unknown), typically before a 'Delivered' status. A soft bounce (often categorized as a 'Block' by SendGrid) is a temporary issue like a full mailbox, where the email is initially accepted then later rejected asynchronously.

How do mailbox full errors affect my sender reputation?

Repeated mailbox full errors, even if temporary, indicate poor list hygiene to ISPs and mailbox providers. Consistently sending to these addresses can negatively impact your sender reputation , potentially leading to more emails being filtered to spam or blocked entirely.

What actions should I take when I see persistent 'mailbox full' blocks?

You should consider suppressing these addresses after multiple occurrences. Regularly cleaning your list and potentially attempting re-engagement through other channels are crucial steps to maintain good email deliverability .

Is it common for email services to accept then reject messages?

Yes, this is a common part of email architecture. Recipient servers often accept messages initially (250 OK) and then perform deeper content scanning or check mailbox quotas before sending an asynchronous rejection if issues are found.

Why does Sendgrid show a delivered status before a block for mailbox full errors? - Troubleshooting - Email deliverability - Knowledge base

I've seen many email senders scratch their heads over SendGrid's delivery reports. It can be confusing when you see an email marked as "Delivered" only for it to be followed by a "Block" event, especially for something as common as a mailbox full error. This seemingly contradictory status often leads to questions about what's truly happening behind the scenes and if the email actually reached its destination.

The core of this confusion lies in how email service providers (ESPs) like SendGrid interpret and report the different stages of email transmission. The "Delivered" status in SendGrid often signifies a specific point in the email journey, not necessarily the final inbox placement.

Understanding SendGrid's event definitions

When SendGrid reports an email as "Delivered", it means the recipient's mail server has accepted the message from SendGrid's servers. This acceptance is typically indicated by a 250 OK SMTP response code. At this stage, the receiving server has taken responsibility for the email, and SendGrid's part of the immediate transfer is complete. This initial acceptance is a crucial step in email delivery, but it isn't the end of the line.

However, after this initial acceptance, the recipient's mail server performs its own internal processing. This post-acceptance phase is where various policies and checks are applied, including spam filtering, content analysis, and mailbox quota verification. If, during this internal processing, the recipient server determines the email cannot be placed in the inbox, it may generate an asynchronous notification back to SendGrid.

This is where the "Block" event often comes into play, particularly with "mailbox full" scenarios. A mailbox full error (SMTP 552) is a soft bounce. While it's a permanent failure for that specific moment, it indicates a temporary condition that might resolve later, unlike a hard bounce (e.g., recipient unknown), which is a permanent failure. SendGrid categorizes such transient, post-acceptance failures as "Blocks" to differentiate them from immediate rejections (hard bounces) that occur during the initial SMTP conversation.

This distinction is vital for understanding deliverability. A "Delivered" event followed by a "Block" (or "bounce" in general terms) doesn't mean SendGrid made a mistake. Instead, it reflects the multi-stage nature of email delivery, where initial acceptance by the recipient server doesn't guarantee final placement, especially if the mailbox is at capacity or other internal rules trigger a subsequent rejection.

Delivered does not mean in inbox

When you see a "Delivered" status in

SendGrid250 OK message, accepting the email for further processing. This is a common point of confusion. The email has been successfully handed off, but it has not yet necessarily landed in the recipient's inbox, spam folder, or any other specific location. Final inbox placement is determined by the recipient server's internal rules after acceptance.

The timing of these events is key to understanding why "delivered" might precede a "block" (or soft bounce). During the initial SMTP conversation between SendGrid and the recipient server, if the server immediately rejects the email (e.g., "user unknown"), SendGrid logs a hard bounce, and you'd typically only see a "Bounce" event without a preceding "Delivered" status. This is a synchronous rejection, happening in real-time during the hand-off.

However, many issues, including a full mailbox, result in asynchronous rejections. The recipient server might initially accept the email, returning a 250 OK code, implying it will attempt delivery. Only later, when it tries to place the email in the recipient's mailbox, does it discover the mailbox is full. At this point, it sends a Non-Delivery Report (NDR) or a bounce message back to SendGrid. This secondary notification is what SendGrid interprets as a "Block" because it's a post-acceptance failure.

This multi-step process is a standard part of how email works, allowing recipient servers to perform more in-depth scanning and policy application after the initial acceptance. It's a system designed for flexibility and robust filtering, but it can create seemingly contradictory status updates for senders if they don't understand the nuances.

The asynchronous nature of email delivery

Synchronous rejections

During the initial SMTP conversation, the recipient server immediately rejects the email, often with a 5xx SMTP code. There's no temporary acceptance.

Example

Invalid recipient addresses (e.g., user@example.com doesn't exist) often lead to immediate hard bounces.

SendGrid status

Bounce: Recorded directly without a prior 'Delivered' event, as the email was never accepted.

Asynchronous rejections (blocks)

The recipient server initially accepts the email with a 250 OK response. Post-acceptance, internal processing identifies an issue, and a Non-Delivery Report (NDR) is sent back later.

Example

Mailbox full errors, some content filtering blocks, or internal transport rules can trigger these delayed rejections. See Twilio SendGrid's deliverability guide.

SendGrid status

Delivered then Block: The email is initially marked 'Delivered' (accepted), followed by a 'Block' (or soft bounce) event when the NDR is received.

Deciphering mailbox full errors and their impact

A "mailbox full" error, specifically, falls into the category of a soft bounce or transient error. Unlike a hard bounce, which suggests a permanent problem with the recipient address, a soft bounce indicates a temporary issue. The recipient's mailbox might be temporarily over quota, or there could be a temporary server issue preventing new mail from being stored.

While SendGrid's system records an initial "Delivered" event because the receiving server said "OK" at the time of hand-off, the subsequent "Block" due to mailbox full is an essential signal. It tells you that even though the email was accepted, it ultimately failed to reach the inbox. This is why it's critical to look beyond just the initial "Delivered" status and analyze all associated events, especially "Blocks" (or soft bounces).

Repeated mailbox full errors from the same recipient can negatively impact your sender reputation, even if they are technically soft bounces. Internet Service Providers (ISPs) and mailbox providers, including

Google and Yahoo, view consistent issues, even temporary ones, as indicators of a sender not maintaining a clean list or engaging in practices that lead to a high volume of problematic recipients. You can learn more about how Gmail 'mailbox full' bounces affect deliverability.

I often see marketers focusing solely on hard bounces, neglecting soft bounces like mailbox full. However, these soft bounces are crucial for list hygiene and maintaining a healthy sender reputation. If you're consistently hitting full mailboxes, it's a strong sign that those addresses are either inactive, abandoned, or have storage issues, and continuing to send to them is detrimental.

SMTP 250 OK Response (Delivered Status)

250 2.0.0 OK: message accepted for delivery

SMTP 552 Response (Mailbox Full Block)

552 5.2.2 Mailbox is full / Quota exceeded

Addressing mailbox full blocks (or blocklists) effectively is crucial for maintaining good email deliverability and sender reputation. Simply ignoring them because they aren't "hard bounces" is a common pitfall that can lead to broader issues, including higher spam complaint rates and more emails landing in the spam folder.

When you encounter persistent mailbox full errors for a specific recipient, it's a clear signal that the address may no longer be active or regularly monitored. Continuing to send to such addresses can inflate your bounce rates and signal to mailbox providers that your list quality is poor. Regularly cleaning your email list is a fundamental aspect of good deliverability practice. We've written extensively on the importance of an in-depth guide to email blocklists and why your emails go to spam.

Reason	SMTP Code Example	SendGrid Status	Recommended Action
Mailbox Full	552 5.2.2	Delivered then Block	Suppress after multiple occurrences or re-engage.
Greylisting	421 / 451	Deferred / Block	Most ESPs handle retries. Monitor for persistent issues.
Recipient Server Busy	450 / 451	Deferred / Block	ESPs retry. Monitor if frequent, indicating potential blocklist (blacklist) issues.
Content Filter	Varies (often 5xx post-acceptance)	Delivered then Block	Review email content for spam triggers. Refer to SendGrid's bounce classifications.

One effective strategy is to implement an automated suppression process for recipients who repeatedly trigger mailbox full errors over a defined period. This prevents you from continuously sending emails that won't be delivered, saving resources and protecting your sender reputation. Another approach is to re-engage these contacts through alternative channels, if available, before removing them from your active sending list.

Views from the trenches

Best practices

Implement two-click unsubscription processes to reduce false positive unsubscribes and maintain list accuracy.

Regularly clean your email list by suppressing recipients who consistently trigger soft bounces, such as mailbox full errors.

Always analyze specific error codes and non-delivery reports (NDRs) from your ESP for clearer insights into failures.

Common pitfalls

Ignoring soft bounces, like mailbox full errors, can significantly harm your sender reputation over time.

Failing to differentiate between synchronous and asynchronous rejections, leading to confusion about email delivery status.

Expecting mailbox providers to alter their internal processing logic or provide ETAs for temporary recipient issues.

Expert tips

For persistent mailbox full errors, consider initiating re-engagement campaigns through alternative channels before fully suppressing the contact from your mailing list.

Focus on actively improving overall list hygiene as a proactive measure to significantly reduce the occurrence of soft bounces and enhance deliverability.

Systematically utilize your ESP's analytics to continuously monitor bounce and block classifications, which helps identify trends and potential deliverability issues early on.

Expert view

Expert from Email Geeks says that ESP summaries often lack sufficient detail to fully diagnose delivery issues, making it challenging to understand the underlying causes without further investigation.

2024-06-24 - Email Geeks

Expert view

Expert from Email Geeks says that it is perfectly common for 5xx errors to occur at various stages of the email transaction, triggered by different underlying reasons.

2024-06-24 - Email Geeks

Navigating email delivery statuses

Understanding why SendGrid shows a "Delivered" status before a "Block" for mailbox full errors boils down to recognizing the layered process of email delivery. The initial "Delivered" event confirms the receiving server has accepted the message, fulfilling SendGrid's immediate responsibility. The subsequent "Block" event reflects a later, asynchronous rejection by the recipient's system, often due to a temporary condition like a full mailbox.

For email marketers, this means looking beyond surface-level statuses and delving into the details of soft bounces. Proactive list hygiene, strategic suppression of problematic addresses, and a clear understanding of SMTP responses are essential for maintaining a healthy sender reputation and achieving optimal inbox placement. By deciphering these seemingly contradictory signals, you can fine-tune your email strategy and ensure your messages reach their intended audience more effectively.