Blocked Emails

What are Blocked Emails?

Blocked emails are messages that a receiving server refuses to accept for reasons unrelated to the validity of the recipient. The mailbox exists, but the server does not want your specific message, IP, or domain delivering to it. Blocks are a subtype of bounce with very different remediation.

Why it matters

A block means someone on the receiving side, a spam filter, a blocklist provider, or a policy engine, has decided your mail is unwanted. Unlike invalid addresses, which are a list problem, blocks are a reputation or content problem that applies across your whole sending stream at that provider. Deliverability engineers and ops teams should treat blocks as a priority signal and investigate root causes.

How it works

When a receiving server blocks a message, it responds with a 5xx code and a textual reason, often including the name of the blocklist or policy triggered. Common examples include "554 5.7.1 Blocked by Spamhaus" or "550 5.7.0 Reject, spam content detected". The sending platform records the block, sometimes retries briefly, and surfaces the reason in reporting so the operator can take action.

Examples

• A campaign blocked at an entire ISP because the sending IP was listed on Spamhaus
• An outage where a content filter blocked mail containing a specific phrase
• A transactional stream blocked at Microsoft after a sudden volume spike

Best practices

• Monitor blocklist status for every sending IP daily
• Investigate block reasons immediately instead of retrying blindly
• Keep content clean of spammy phrases and excessive links or images
• Use Sendkit's reputation monitoring to detect blocks before users complain

FAQs

How is a block different from a hard bounce?

A hard bounce means the mailbox does not exist. A block means the mailbox exists but the receiver is refusing your message. Fix blocks by fixing reputation or content, not by removing addresses.

How do I get off a blocklist?

Most blocklists provide a delisting form. Fix the underlying issue, then request removal. Expect 24 to 72 hours for the change to propagate.

Can content alone cause a block?

Yes. Attachments, link shorteners, phishing-like phrases, and poorly formatted HTML can trigger content filters even for well-reputed senders.