Understanding Blocks

What is a block?

A block is a type of non-delivery to a recipient (email address/member) due to the recipient’s mail server rejecting the message. A message can be blocked for one of many reasons. Blocks are completely different than bounces, which are “bad” addresses to some degree. Blocks do not necessarily indicate a bad address at all, but for whatever reason during the send attempt, JangoMail was not allowed to deliver the email because the receiving server basically said NO.

What should you do with these blocks?

Blocks should be treated very differently from bounces. These addresses should NOT be removed from your list(s), but should be retried after several hours from the original send. Currently JangoMail does not automatically retry blocks, however this is something we hope to have in the near future. Blocks are usually temporary in nature, although some can be permanent if the underlying reason for the block is not addressed (e.g., a blacklisted IP address), and are removed all on their own from within the Reports interface (i.e., you can’t add or remove them like the way you can with unsubcribes or bounces). We monitor our sending IP addresses and actively request removal of blocks as they appear in the logs.

How are blocks caused?

  • IP Address Reputation
    • Whether you are sending using a dedicated IP address unique to your account or a shared group of IP addresses used by multiple accounts, each and every IP address has its own reputation. IP address reputation is affected by all of the below factors. Email messages can be blocked due to IP address reputation, which our delivery teams works hard on to combat/prevent on a daily basis.
  • Domain/From Address Reputation
    • Like IP address reputation, a domain/FROM address can hold its own reputation as well. This reputation can be affected by the other factors on this list or based on the domain/DNS setup alone. If a domain gets blocked, it’s the responsibility of the domain owner to request removal of any blocks or blacklists from ISPs.
      • Lack of DNS records
        • If there are missing DNS records for the sending domain (SPF, DKIM, CTD, DMARC) the messages can be blocked. A missing reverse listing (rDNS) or mail exchanger (MX) record at the domain can cause a block (infrequently seen, but it does happen).
      • Invalid Domain
        • Sending from an invalid from address (domain) can result in blocks, delivery issues. You should always send from a valid email address, at a valid business domain that you/your company owns.
  • Message Content
    • The content of the message itself can cause blocks that are unrelated to any of the previous factors listed above. Wording, images, URL’s, and layout of the message can cause a server to block your messages. A high spam score is generally the reason here.
  • Sending Speed
    • Many servers, both public and private, have limits as to how many emails they can and will accept in a set time frame. If they start to receive more emails than their limits allow, they can start blocking subsequent emails. These blocks can last for minutes or hours, depending on their own rules. JangoMail has internal rate limits for public domains like Gmail, Yahoo, Hotmail, and more. We monitor the diagnostic codes we get from their servers to dictate how slow or fast our sending speeds should be. ISP’s do not post their sending rates, so it is an ever-changing rate that we work to keep on top of.
  • Previous User Activity
    • How your recipients react to your email messages plays a huge part into future email deliveries. Users that mark your messages as Junk or Spam (these come into JangoMail as complaints) can have a negative impact on your future sending attempts. It is highly recommended to have an unsubscribe link at the top of your emails, not just at the bottom. This gives users an easy option to opt out of future emails. Unsubscribes do not have a negative impact on your delivery.

Viewing reports

Blocks are visible in your Reports dashboard/grid. Blocks are kept in their own column, right after complaints. You can click on the block number and view each and every address that was blocked and the reason why it was blocked.

Understanding the codes

Every server can categorize their block code however they see fit. Because of this, it is impossible to outline every code and what it means. Below is a list of common blocks we see and their diagnostic codes.

  • IP Address Reputation
    • "553 5.7.1 [TSS07] Connections will not be accepted from (IP Address), because the ip is in (Providers) list; see (URL of Provider)"
    • For assistance forward this email to (email address)
    • "421 Too many spams from your IP (IP Address), please visit (URL)
    • 554 5.1.0 Sender denied
  • Domain/From Address Reputation
    • 553 Sorry, your email address (email address) has been
    • 554 5.1.0 Sender denied
      • Lack of DNS records
        • 521 5.2.1 : (DMARC:F1) This message failed DMARC Evaluation and is being refused due to the policy provided by the From domain.
        • "550 5.1.0 (IP Address) is not allowed to send from per it's SPF Record. Please inspect your SPF settings, and try again. IB508 "
        • 553 SPF (Sender Policy Framework) domain authentication
        • 550 Message was blocked by server (failed SPF)
        • 550 5.7.0 Message rejected per SPF policy
      • Invalid Domain
        • 550 5.1.0 Invalid sender domain
  • Message Content
    • 554 5.7.1 [P4] Message blocked due to spam content in the message.
    • 554 5.7.1 The message from (from address or sometimes blank) with the subject of (Subject line) matches a profile the Internet community may consider spam. Please revise your message before resending.
    • 554 rejected due to spam URL in content
    • 550 5.7.1 Message rejected as spam by Content Filtering.
  • Sending Speed
    • 421 PR(ct1) (SNT004-MC6F10) Unfortunately, some messages from (IP Address) weren't sent. Please try again. We have limits for how many messages can be sent per hour and per day. You can also refer to (URL).
  • Previous User Activity
    • 421 4.7.0 [TSS04] Messages from (IP Address) temporarily deferred due to user complaints -; see (URL of blocking provider)


Deferred messages

Some messages are deferred, which is a type of block. For deferrals (typically seen in SMTP logs with a 421 response code), we use a retry process to try and complete the delivery process. Currently, we will retry a message 4 times, with increasing in-between retry times. The times are currently set to 15, 30, 60 and then 240 minutes. This means you may not have a definitive delivery status for almost 6 hours if the message has been deferred. Many messages are successfully accepted for delivery on the first or second attempts, and a smaller proportion thereafter. After the last attempt, the message is recorded as a block in your account.

Re-sending to blocked recipients

In the simplest case for broadcast/mass email campaigns, you can download the blocked email addresses as a report, filter on the reason(s) to get the addresses that warrant a re-send attempt, upload the addresses as list within your account, and re-send the campaign message to those recipients. If your original message used personalization, extra work will be involved (contact Support for assistance in this case). If the blocks were due to an IP issue, contact Support before sending. We may have already cleared the block, or we can send your messages out on a different IP address (considering your SPF record as well).

For a transactional message, those will have to be triggered on your end as we do not keep the completed message.

We’re working on preserving blocked messages so that they can be automatically or manually moved to a different sender for a subsequent send attempt. These messages would be more for an IP/rate limiting issue as opposed to a spam/blocked content issue.


Blocks can be a very helpful tool in your email success, giving you insight into your overall delivery. As always, please contact us with any questions you might have. We want you to feel comfortable with reviewing and understanding your Block reports.

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