Summary
Sendgrid's reporting of a 'Delivered' status followed by a 'Block' (specifically due to a 'mailbox full' error) stems from the fundamental asynchronous nature of email delivery, and the SMTP protocol governing email exchange. The 'Delivered' event in Sendgrid signifies that the receiving server acknowledged and accepted the email for delivery. However, final delivery might fail later due to several reasons. This failure manifests as a 'Block' event. Common causes include the recipient's mailbox being full, quota issues, server policies, or deferred delivery configurations on the recipient's mail server. Key to understanding this process is to interpret Sendgrid's 'Delivered' as 'Accepted' rather than a guarantee of successful delivery to the inbox. A 'Block' event is often transient and reflects a non-permanent delivery issue, especially 'mailbox full'. Repeated 'mailbox full' errors for an address warrant suppression to protect sender reputation. This two-step process is a standard practice in email handling, with the receiving mail server often accepting emails initially and determining delivery failures later.
Key findings
- Asynchronous Communication: Email communication is asynchronous; the receiving server accepts the message first and determines deliverability later.
- 'Delivered' Means 'Accepted': In Sendgrid, 'Delivered' signifies that the receiving server acknowledged the message but *not* that it reached the inbox.
- SMTP Protocol: The SMTP protocol permits initial acceptance (250 OK) followed by a later non-delivery due to various reasons.
- Transient Blocks: 'Block' events often represent transient issues, like a full mailbox, which might resolve on their own.
- Deferred Delivery: Receiving servers may defer final delivery, leading to a 'Delivered' status followed by a later 'Block'.
Key considerations
- Interpret Statuses Correctly: Distinguish 'Accepted' (Sendgrid 'Delivered') from successful inbox delivery.
- Address Suppression: Implement a suppression strategy for addresses with repeated 'mailbox full' errors.
- Monitor Block Reasons: Monitor the reasons behind 'Block' events to differentiate transient issues from more serious deliverability problems.
- Analyze Bounce Types: Understand the difference between soft and hard bounces and implement appropriate handling strategies.
- Treatments of blocks: If a bounce happens 7 seconds after delivery with a mailbox full, it is not a blocking issue and is standard.
What email marketers say
9 marketer opinions
Sendgrid's practice of showing a 'Delivered' status before a 'Block' (due to issues like a full mailbox) reflects the asynchronous nature of email communication. The 'Delivered' status indicates that the receiving server accepted the email. The 'Block' status signifies a later determination by the receiving server that the email could not be delivered, often due to issues that aren't immediately apparent upon initial acceptance, such as the recipient's mailbox being full. 'Delivered' in Sendgrid's terminology should be interpreted as 'Accepted'.
Key opinions
- Asynchronous Bounce: The 'Block' event for mailbox full is a post-acceptance, asynchronous bounce. The recipient server accepts the message and later asynchronously generates a bounce message.
- Delivery vs. Acceptance: Sendgrid's 'Delivered' status really means 'Accepted'. The recipient server acknowledged receipt of the email, but delivery isn't guaranteed at this point.
- Normal Process: This two-step process (delivered followed by block) is a normal part of email deliverability and reflects how email servers handle temporary delivery failures.
- Non-Permanent Block: The 'Block' status usually indicates a non-permanent failure, like a full mailbox, and not necessarily a permanent delivery issue.
Key considerations
- Terminology: Understand that Sendgrid's use of 'Delivered' means 'Accepted,' and a 'Block' doesn't always signify a severe deliverability problem.
- Asynchronous Nature: Email events are inherently asynchronous. Receiving servers can report issues after initially accepting the message.
- Interpreting Blocks: A 'Block' often indicates a temporary issue like a full mailbox that may resolve itself later. Analyze the type of block to determine the correct course of action.
- Mailbox full: If you get multiple Mailbox Full errors in a row you should suppress the account from recieving mail.
Marketer view
Email marketer from Quora shares that Sendgrid uses 'Delivered' to mean the email made it to the recipient's server, and 'Block' to indicate a non-permanent delivery failure. Both statuses are part of a normal delivery attempt.
29 Apr 2025 - Quora
Marketer view
Email marketer from Stackoverflow responds that email events are inherently asynchronous, with receiving servers reporting issues (like mailbox full) *after* initially accepting the message.
30 Dec 2024 - Stackoverflow
What the experts say
5 expert opinions
Sendgrid's practice of showing a 'Delivered' status before a 'Block' (particularly due to mailbox full errors) reflects the asynchronous nature of email delivery. A 'Delivered' status in Sendgrid signifies that the recipient server accepted the email, which should be interpreted as 'Accepted' rather than actual delivery. A subsequent 'Block' indicates a deferred or asynchronous bounce, where the receiving server later determines it cannot deliver the message due to constraints like a full mailbox. This sequence is a standard part of email protocol, not necessarily indicative of a severe issue, and asynchronous bounces are permitted. Multiple 'Mailbox Full' errors in a row suggests suppressing the address from future sends.
Key opinions
- Asynchronous Delivery: Email delivery is often asynchronous; the receiving server may initially accept the message and only later determine it cannot be delivered.
- Delivery vs. Acceptance: Sendgrid's 'Delivered' status actually means the receiving server 'Accepted' the message, not that final delivery was successful.
- Deferred Bounce: A 'Block' following a 'Delivered' status typically indicates a deferred bounce due to conditions like a full mailbox.
- Standard Protocol: This sequence of 'Delivered' followed by 'Block' is standard email protocol and doesn't automatically indicate a problem.
Key considerations
- Interpret 'Delivered': When using Sendgrid, interpret 'Delivered' as 'Accepted,' recognizing that the email might still bounce later.
- Asynchronous Nature: Be aware that email events are asynchronous and servers may report delivery failures after initial acceptance.
- Mailbox Full Handling: Implement logic to suppress email addresses that repeatedly return 'Mailbox Full' errors to improve sending reputation.
- Analyze Block Reasons: Investigate specific block reasons to determine if the issue is temporary (e.g., mailbox full) or more serious (e.g., spam filter).
Expert view
Expert from Word to the Wise (Laura Atkins) explains that the delivered status prior to the block indicates a 'deferred bounce'. The receiving server initially accepts the message, and only determines later that it cannot be delivered due to issues like mailbox being full. This is standard protocol of email and is not abnormal.
2 Mar 2025 - Word to the Wise
Expert view
Expert from Email Geeks explains that if a bounce happens 7 seconds after delivery with a mailbox full, it is not a blocking issue. Suggests suppressing the account from receiving mail if multiple Mailbox Full errors occur in a row.
23 Feb 2023 - Email Geeks
What the documentation says
4 technical articles
The 'Delivered' status followed by a 'Block' (specifically for mailbox full) in Sendgrid is due to the standard SMTP protocol and the way email servers handle temporary delivery failures. When Sendgrid hands off an email and receives a 250 OK, it registers a 'Delivered' event. However, a subsequent 'Block' event signifies a later non-delivery, which can occur because the receiving server initially accepts the message but later determines it cannot be delivered due to issues like a full mailbox, quota limits, policy reasons, or deferred delivery settings. This sequence is a normal part of the email delivery process, especially with soft bounces.
Key findings
- SMTP Standard: The SMTP protocol (RFC 5321) allows for initial message acceptance (250 OK) followed by a later delivery failure notification.
- Deferred Delivery: Email servers like Microsoft Exchange can defer delivery due to temporary conditions, leading to a 'Delivered' status followed by a 'Block' later.
- Soft Bounce Handling: Soft bounces (e.g., mailbox full) can occur after initial acceptance, resulting in a 'Delivered' status (acceptance) followed by a soft bounce (block).
- Sendgrid Event Types: Sendgrid's 'Delivered' event signifies that the email was handed off to the recipient server with a 250 OK; 'Block' indicates a later non-delivery.
Key considerations
- Protocol Understanding: Recognize that the 'Delivered' status doesn't guarantee final delivery; the email can still bounce later due to temporary issues.
- Server Configurations: Be aware that receiving servers may implement deferred delivery or have quota limits that cause delivery failures after initial acceptance.
- Bounce Type Analysis: Analyze the type of bounce (soft vs. hard) to determine the appropriate action. Soft bounces might resolve themselves, while hard bounces require address removal.
- Monitoring: Monitor 'Block' events and understand the reasons behind them to optimize email sending practices and maintain deliverability.
Technical article
Documentation from Microsoft explains that Microsoft Exchange allows for 'deferred delivery.' The server can initially accept an email, but delay the final delivery due to temporary conditions (like mailbox full). This would result in an initial 'Delivered' status, followed later by a 'Block'.
1 Sep 2024 - Microsoft
Technical article
Documentation from Sendgrid explains the different event types, the 'delivered' event means that Sendgrid handed off the email to the recipient's mail server and received a 250 OK response. A separate 'block' event indicates a non-delivery, which can happen later due to reasons like 'mailbox full'.
23 Sep 2023 - Sendgrid
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