Browse the Learning Center
Delivery errors
SMTP 550s, TLS failures, and every Gmail and Yahoo error code.
34 guides
All Delivery errors guides
Gmail SMTP settings: ports, auth, and alignmentCompare Google's three Gmail and Workspace SMTP methods, pick the right authentication model for your device, and verify SPF, DKIM, DMARC, and delivery.
Office 365 SMTP settings and authenticationUse current Office 365 SMTP settings, choose OAuth and mailbox policy safely, then validate SPF, DKIM, DMARC, safe sending limits, and delivery.
ESMTPS: What It Means in an Email HeaderESMTPS in an email header means ESMTP crossed a TLS-protected hop. Learn how it differs from ESMTP, ESMTPSA, STARTTLS, and implicit TLS connections.
550 5.7 0 DMARC policy violation550 5.7 0 DMARC policy violation needs provider-specific evidence. Check the exact SMTP response, sending domain, and published DMARC record.
550 message rejected due to senders DMARC policy550 message rejected due to senders DMARC policy requires the failed message, recipient response, and DNS evidence before a safe repair for your domain.
Bounce email address: what it is and how to find itBounce email address explained: identify the envelope return path, read delivery-status evidence, repair the confirmed cause, and retest the same route.
Email list bounce checker: how to interpret and act on resultsEmail list bounce checker results help separate bad, deliverable, and uncertain addresses before sending, then guide list cleanup and retesting.
SMTP and POP3 difference: sending versus retrieving emailSMTP and POP3 difference: SMTP sends and relays email, while POP3 retrieves messages from a server to a client mailbox.
SMTP header analyzerSMTP header analyzer workflow: paste raw email headers, interpret delivery, SPF, DKIM, and DMARC evidence, then retest the right layer today.
TLS-RPT reporting: how to read SMTP TLS reportsTLS-RPT reporting explains the JSON reports sent for SMTP TLS failures, their result types, delivery format, cadence, and how to interpret them.
TLS-RPT RFC: SMTP TLS Reporting explainedTLS-RPT RFC 8460 defines SMTP TLS Reporting, its DNS policy record, report endpoints, report format, and practical validation checks for recipient domains.
What causes an email to bounce?What causes an email to bounce? Preserve the bounce evidence, validate undeliverable addresses, correct list data, and retest the same path.
What are Gmail and Yahoo error codes and how can I fix them?Gmail and Yahoo error codes explain a temporary or permanent SMTP failure. Preserve the full response, identify its cause, repair it, and retest.
How do you fix 'Reverse DNS does not match SMTP banner'?Reverse DNS does not match SMTP banner means the tested server greeting and PTR hostname differ. Compare the route, repair the owning layer, and retest.
Email rejected per DMARC policy: how to fix the bounceEmail rejected per DMARC policy means no aligned SPF or DKIM pass was found. Trace the sender, repair alignment, and retest the same path today.
Why does Gmail report TLS errors?Gmail TLS errors mean an SMTP connection could not meet transport-security checks. Capture the exact host, handshake evidence, policy, then retest.
Microsoft SMTP 5.4.1 Relay Access DeniedDiagnose Microsoft SMTP 5.4.1 Relay Access Denied from the full NDR. Identify the rejecting host, check destination acceptance and connectors, then retest.
What does SMTP connection refused mean?SMTP connection refused means a TCP connection was rejected before SMTP began. Diagnose the endpoint, listener, firewall, DNS, and retry path.
Soft bounce vs hard bounce: what each means and what to doSoft bounce vs hard bounce: learn the temporary and permanent delivery difference, how ESP labels vary, and how to respond safely with evidence.
SMTP vs IMAP vs POP3: what's the difference?SMTP vs IMAP vs POP3: SMTP sends mail, while IMAP syncs server mail and POP3 downloads it. Learn the secure ports and validation steps clearly.
What is MTA-STS and how does it secure SMTP with TLS?MTA-STS is an SMTP transport policy that requires supporting senders to use authenticated TLS with a domain's authorized MX hosts. Learn how to deploy it.
Bounce back email Outlook: diagnose and fix the NDRBounce back email Outlook errors are non-delivery reports. Read the exact SMTP code and diagnostic text, apply the narrow repair, then retest.
Difference between IMAP and SMTPIMAP retrieves and manages mail stored on a server; SMTP transfers and submits outgoing mail between systems, per RFC 9051, RFC 5321, and RFC 6409.
Email bounce rate: how to calculate and investigate itEmail bounce rate is bounced messages divided by your ESP's stated denominator. Separate hard and soft bounces, inspect reasons, and retest.
Why would an email bounce?Why would an email bounce? Preserve the original delivery notice, identify the sending path and provider, then use provider evidence to choose a repair.
Bounce-back email: how to read the error and fix the causeBounce-back email errors identify a delivery failure. Read the SMTP status, repair the confirmed cause, and retest the same sending path now.
Authentication failed email: fix mailbox and SMTP login errorsFix authentication failed email errors by checking mailbox sign-in, SMTP AUTH credentials, TLS, and sender permissions.
What does no MX record found mean in a bounce?No MX record found in a bounce can mean no usable mail route, a null MX, DNS failure, or a bad address. Diagnose the DNS result and retest safely.
What does SMTP 550 permanent failure mean?SMTP 550 permanent failure means a receiving server rejected an SMTP action. Use the enhanced code, response text, and command stage to diagnose it.
Why is Microsoft 365 rejecting my email with a 550 5.7.x error?Microsoft 365 returns 550 5.7.x when your mail fails a policy or authentication check. Fix SPF, DKIM, and DMARC alignment to clear the access-denied bounce.
What does SMTP error 421 mean and how do I fix it?SMTP error 421 is a temporary deferral, not a bounce. Fix it by slowing your send rate and authenticating mail with SPF, DKIM, and DMARC before retrying.
Why is Gmail rejecting my emails with a 550 error?Gmail 550 5.7.26 rejections mean your mail fails authentication. See why Google now rejects instead of defers, and the exact fixes to get delivered.
How can you resolve Yahoo and Gmail email error codes?Google and Yahoo are tightening email authentication rules, and you may see new SMTP error codes when sending messages.
What is SMTP? Simple Mail Transfer Protocol ExplainedWhat SMTP is and how the protocol moves email between servers, with the ports, commands, and the SPF, DKIM, and DMARC setup that keeps your mail trusted.