Pillar · 43 errors
Email & Deliverability
If your domain’s email lands in spam or bounces, the cause is almost always a DNS authentication record — SPF, DKIM or DMARC — that’s missing or misconfigured.
554 5.7.1554 5.7.1 Client Host Blocked Using Spamhaus The receiving server refused your connection because your sending IP is listed on a Spamhaus DNSBL. critical 550 5.7.509550 5.7.509 Access Denied, Sending Domain Does Not Pass DMARC (Exchange Online) Exchange Online rejected the message because the sending domain failed DMARC and publishes a p=reject policy. critical 550 5.7.515Outlook 550 5.7.515 Access Denied (Bulk Sender Requirements) Outlook. critical 550 5.7.25550 5.7.25 Reverse DNS / PTR Record Failure The receiving server rejects the connection because the sending IP has no PTR record or it fails forward-confirmed reverse DNS. critical 550 5.7.1 [BL] / OU-002550 5.7.1 Outlook/Hotmail Sending IP Blocked (Reputation Blocklist, OU-002) Outlook. critical 550 5.7.1550 5.7.1 Message rejected by policy (DMARC/SPF/DKIM or blocklist) The receiving server accepted the connection but refused the message on policy grounds — authentication failure or a bad sender reputation. high dmarc=failDMARC fail (p=reject / p=quarantine) — message rejected or sent to spam Mail failed DMARC because neither SPF nor DKIM passed with alignment to the From: domain, so the receiver bounced or spam-foldered it. high 550 5.7.1550 5.7.1 Unable to Relay The receiving server refuses to relay your message because the connection isn't authenticated or the recipient isn't a local domain. high 421 4.7.28421-4.7.28 Gmail Unusual Rate of Unsolicited Mail (IP Rate Limited) Gmail is temporarily deferring your mail after flagging an unusual volume of unsolicited mail from your IP or domain. high 550 5.7.26550-5.7.26 Unauthenticated Email Not Accepted (Gmail) Gmail rejected your mail because it passed neither aligned SPF nor DKIM, violating Google's bulk-sender authentication rules. high 550 5.7.708550 5.7.708 Access Denied, Traffic Not Accepted From This IP (Microsoft 365) Microsoft 365 blocked your outbound mail because the sending IP has low reputation, common on new or trial tenants. high dkim=fail (bh)DKIM Fail: Body Hash Did Not Verify (bh= Mismatch) DKIM failed because the receiver's recomputed body hash does not match the signed bh= tag — the body changed after signing. high 550 5.1.1550 5.1.1 Email Account Does Not Exist (Gmail gsmtp) Gmail rejected the message because the mailbox before the @ does not exist on an otherwise valid domain. high SPF PermErrorSPF PermError: Too Many DNS Lookups Your SPF record triggers more than 10 DNS-querying mechanisms, so evaluators return PermError and SPF fails for every message. high 550 5.7.1 (S3150)Outlook/Hotmail "550 5.7.1 ... blocked (S3150)" sending IP throttled Microsoft throttled your sending IP for reputation reasons; S3150 is a soft throttle, S3140 is a hard block. high 421-4.7.28Gmail "421-4.7.28 ... has exceeded its quota for sending" (IP rate limited) Gmail is throttling your IP or domain for sending too much, too fast, and defers further mail. high 550 5.7.350Outlook "550 5.7.350 Remote server returned message detected as spam" Microsoft's outbound filter judged your message as spam and refused to relay it, bouncing it back to you before it ever reached the recipient. high 553 5.1.8SMTP "553 5.1.8 Domain of sender address does not exist" The receiving server looked up your envelope-sender domain in DNS, found nothing, and rejected the message as a likely forgery or typo. high 421 4.7.0 [TSS04]Yahoo 421 4.7.0 [TSS04] Temporarily Deferred Yahoo/AOL is throttling, not rejecting, your IP because it looks risky right now — retry later at lower volume. high SPF PermError (2 records)SPF PermError — Multiple SPF Records on One Domain Two v=spf1 TXT records on one domain is invalid, so receivers return PermError and SPF fails on every message. high dkim=permerrorDKIM permerror (no key for signature) A DKIM signature is present but the receiver can't fetch a usable public key at the selector, so the check errors out. high DMARC misalignmentDMARC Fails Even Though SPF/DKIM Pass (Alignment Failure) SPF and DKIM pass on the ESP's own domain but neither aligns with your visible From:, so DMARC fails. high 550-5.7.1 (unsolicited)Gmail 550-5.7.1 Message Blocked as Likely Unsolicited Mail Gmail's spam filter judged the message likely unsolicited and blocked it on reputation or content grounds. high SPF NoneSPF None — No SPF Record Found No v=spf1 TXT record exists for your domain, so SPF has no policy to evaluate and returns None. high 535 5.7.8535 5.7.8 Username and Password Not Accepted (Gmail SMTP) Gmail's SMTP server rejects your login because it no longer accepts a plain account password. high 535 5.7.139535 5.7.139 Authentication Unsuccessful — SMTP AUTH Disabled (Microsoft 365) Microsoft 365 refuses the SMTP login because Authenticated SMTP is turned off for the mailbox or tenant. high 550 5.4.5550 5.4.5 Daily User Sending Quota Exceeded (Gmail / Workspace) The account hit Gmail's rolling 24-hour recipient cap; outbound mail stays blocked until the window clears, with no midnight reset. high 550 5.7.23550 5.7.23 SPF Validation Failed (Exchange Online NDR) The receiving server rejected the message because the sending domain's SPF check returned fail; the sender IP isn't authorized. high 554 5.7.9554 5.7.9 Message Not Accepted for Policy Reasons (Yahoo / AOL) Yahoo permanently rejected the message for failing its bulk-sender policy, usually a DMARC, SPF, or DKIM alignment failure. high List-Unsubscribe (RFC 8058)Missing One-Click List-Unsubscribe Header (Gmail/Yahoo Bulk-Sender Requirement) Bulk mail to Gmail/Yahoo lacks the List-Unsubscribe-Post one-click header, so it gets junked or rejected. high 550 5.1.1550 5.1.1 User unknown / recipient address does not exist A hard bounce: the mailbox in the RCPT TO doesn't exist on the receiving server, so delivery permanently fails. medium 421 4.7.0421 4.7.0 Temporary deferral / too many connections / rate limited A soft bounce: the receiving server is throttling you for sending too fast or opening too many connections, and asks you to slow down and retry. medium 550 5.4.1550 5.4.1 Recipient Address Rejected: Access Denied (Exchange Online) Exchange Online rejected the mail because the recipient address has no matching object in the tenant, or a policy blocked the sender. medium 552 5.2.2552 5.2.2 Mailbox Full / Quota Exceeded A permanent bounce: the recipient's mailbox is over its storage quota and will not accept the message. medium 552 5.3.4SMTP "552 5.3.4 Message size exceeds fixed limit" The message plus its Base64-encoded attachments is larger than the receiving server's maximum, so it's rejected. medium SPF SoftFail (~all)SPF SoftFail (~all) — Mail Marked Suspicious The sending IP isn't in your SPF record, so ~all returns SoftFail — accepted but pushed toward spam. medium 451 4.7.500Outlook 451 4.7.500 Server Busy (Reputation Throttle) Outlook. medium 454 4.7.0454 4.7.0 Cannot Authenticate Due to Temporary System Problem A temporary SMTP AUTH rejection from too many concurrent connections or recent bad-password attempts; back off and retry. medium 552 5.7.0552 5.7.0 Message Blocked — Content Presents a Potential Security Issue (Gmail) Gmail rejected the message because an attachment is a blocked executable type or its virus scan flagged the content. medium DKIM key too shortDKIM Key Too Short — 1024-bit Signature Flagged or Rejected A 1024-bit RSA DKIM key is treated as weak, yielding dkim=neutral/fail and intermittent authentication trouble. medium 451 4.7.1SMTP "451 4.7.1 Greylisting in action, please try again later" Receiving server temporarily rejects mail from an unfamiliar sender so a compliant MTA retries and gets through. low Apple MPPApple Mail Privacy Protection Inflating Open Rates Apple Mail pre-loads your tracking pixel on delivery, so Apple users register as opens whether or not they read the email. low BIMI / VMCBIMI Logo Not Showing in Gmail Despite Valid VMC Gmail won't render your BIMI logo even with a valid VMC, usually because DMARC isn't at enforcement. low