// how to fix
Not sure what is wrong?How to fix email problems
Your test or scan found a problem? Find the matching guide below: what it means, why it matters, and how to fix it step by step.
Bounce 40
DNS 36
DMARC 33
Reputation 33
SPF 30
DKIM 27
Gmail 19
Security 19
Blacklist 18
Content 14
Headers 11
Mailing list 11
Spam score 11
Reverse DNS 10
Microsoft 365 8
Outlook 8
Mail server 7
MX 5
Yahoo 4
TLS 3
Apple 2
Google Workspace 2
BIMI 1
E-commerce 1
GMX 1
WordPress 1
Zoho 1
40 guides · tag Bounce
| Problem | In short | Tags | |
|---|---|---|---|
| Gmail bounce "550 5.7.26": unauthenticated email | Gmail refused your email because it could not confirm it comes from your domain. SPF, DKIM or DMARC is missing or failing. Here is how to fix it. | Fix it | |
| Bounce "550 5.7.1": message rejected or blocked by policy | A 5.7.1 bounce means the receiving server refused your email on purpose. Learn to read bounce messages, find the real reason and fix it. | Fix it | |
| High bounce rate: clean your mailing list | Sending to addresses that no longer exist damages your reputation fast. Learn the difference between hard and soft bounces and remove bad addresses after every send. | Fix it | |
| Emails delayed: greylisting and 4xx temporary errors | Some servers answer "try again later" on purpose. A normal mail server retries and the email arrives a few minutes late. Here is how to tell normal delays from real problems. | Fix it | |
| Yahoo and AOL "421 4.7.0 [TSS04]" deferrals | Yahoo is temporarily refusing your emails because of their volume or complaints. Slow down, keep retrying, and fix what Yahoo does not like. | Fix it | |
| Emails blocked by Orange, SFR, Free or other French ISPs | French internet providers filter strictly with their own rules. Fix your reverse DNS and authentication, slow down, and use their contact procedures. | Fix it | |
| Your emails are not received at all: a step-by-step checklist | Your recipients say nothing arrived, not even in spam. Follow these steps in order to find where the email gets lost. | Fix it | |
| Use double opt-in for your newsletter | Ask every new subscriber to confirm their address with one click. Your list stays clean, bounces and complaints drop, and your emails reach the inbox more often. | Fix it | |
| Emails to Apple "Hide My Email" addresses (privaterelay.appleid.com) | Users who sign up with Apple can hide their real address behind a relay. Your emails reach them only if you register your sending domain with Apple and authenticate it. | Fix it | |
| Bounce "550 5.1.1": the address does not exist | The receiving server says the mailbox does not exist. Check the spelling, remove the address from your lists, and keep your bounce rate low. | Fix it | |
| Bounce "552 5.2.2": the mailbox is full | The recipient has run out of storage space. It is their problem, not yours, but repeated "mailbox full" bounces often mean an abandoned address. | Fix it | |
| "554 5.7.1 Relay access denied" when sending | Your mail server refuses to pass your email on because you are not logged in. Usually a missing SMTP login or the wrong port in your app or printer. | Fix it | |
| Microsoft 365 "550 5.1.8 Access denied, bad outbound sender" | Microsoft 365 has blocked a user from sending because the account sent spam, usually after being hacked. Secure the account, then unblock it. | Fix it | |
| Gmail "421 4.7.28": unusual rate of unsolicited mail | Gmail is slowing down your emails because it sees a sudden spike or spam-like traffic from your IP or domain. Slow down and fix your reputation. | Fix it | |
| Microsoft "451 4.7.500 Server busy": temporary deferral | Outlook.com and Hotmail are holding back emails from your IP because they do not know it or trust it yet. Keep volumes low and build reputation. | Fix it | |
| Gmail bounce "550 5.7.25": reverse DNS (PTR) required | Gmail refuses your email because your sending IP has no reverse DNS, or one that does not point back to the IP. Ask your host to set a proper PTR. | Fix it | |
| Gmail "550-5.7.1 ... likely unsolicited mail": why it happens and how to fix it | Gmail rejected your email as likely spam. Learn what triggers 550-5.7.1 unsolicited mail and how to fix authentication, reputation and content. | Fix it | |
| Gmail "421-4.7.0 ... very low reputation of the sending IP address": how to fix it | Gmail is temporarily deferring your emails because of low IP or domain reputation. Learn why 421-4.7.0 happens and how to rebuild your reputation. | Fix it | |
| Gmail "550-5.7.28 ... unusual rate of unsolicited mail from your IP address": how to fix it | Gmail blocked your IP after detecting an unusual rate of unsolicited mail. Find the source of the spam, stop it and rebuild your reputation. | Fix it | |
| Gmail "552-5.7.0 ... content presents a potential security issue": blocked attachments fixed | Gmail blocked your email because of a risky attachment: executable, script, macro or protected archive. See which files are blocked and how to share them. | Fix it | |
| Gmail "550 5.4.5 Daily user sending limit exceeded": what to do | Your Gmail or Google Workspace account reached its daily sending limit. Learn the limits, how long the block lasts and how to send bulk email properly. | Fix it | |
| Gmail "550-5.7.1 ... is not RFC 5322 compliant": fix missing or duplicate headers | Gmail rejected your email because its headers break the RFC 5322 standard: missing or duplicate From, Date or Message-ID. Here is how to fix it. | Fix it | |
| Outlook.com "550 5.7.1 ... part of their network is on our block list (S3150)": how to fix S3150 and S3140 | Outlook.com and Hotmail refuse your email because your IP or its network range is on their block list (S3150, S3140). Learn how to request delisting. | Fix it | |
| Microsoft 365 "550 5.7.606 Access denied, banned sending IP": how to delist your IP | Microsoft 365 rejects your email because your IP is on its banned list (5.7.606 to 5.7.649). Learn how to use the delist portal sender.office.com. | Fix it | |
| Microsoft "550 5.7.509 ... does not pass DMARC verification and has a DMARC policy of reject": how to fix it | Outlook and Microsoft 365 reject your email because it fails DMARC and your domain uses p=reject. Fix SPF or DKIM alignment to get delivered. | Fix it | |
| Outlook.com "550 5.7.515 Access denied, sending domain doesn't meet the required authentication level": how to fix it | Outlook.com, Hotmail and Live now require SPF, DKIM and DMARC from high-volume senders. Here is how to fix error 550 5.7.515. | Fix it | |
| Microsoft 365 "554 5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException": how to fix it | Microsoft 365 refused to send because the account has no Send As permission for the From address. Fix permissions in Outlook, apps, printers and scripts. | Fix it | |
| Microsoft 365 "550 5.4.1 Recipient address rejected: Access denied. AS(201806281)": what it means and how to fix it | Microsoft 365 rejected your email because the recipient address does not exist in that organization. Learn what senders and admins can do about 5.4.1. | Fix it | |
| Yahoo / AOL "553 5.7.1 [BL21] Connections will not be accepted ... Spamhaus's list" (BL23): how to fix it | Yahoo and AOL refuse your server because its IP is on a Spamhaus list. Find which list, fix the cause, get delisted and resend. | Fix it | |
| Yahoo "554 5.7.9 Message not accepted for policy reasons" and "[PH01]": causes and fixes | Yahoo refused your email for policy reasons: often failed DMARC or authentication, sometimes suspicious content. How to find the cause and fix it. | Fix it | |
| Yahoo "421 4.7.0 [TSS09] All messages from [IP] will be permanently deferred; Retrying will NOT succeed": what to do | Yahoo has stopped accepting mail from your IP because of poor reputation. Why TSS09 happens, how to recover and how to contact Yahoo. | Fix it | |
| iCloud "554 5.7.1 [CS01] Message rejected due to local policy" and "[HM08]": how to fix it | Apple iCloud Mail refused your email with CS01 or HM08. What these codes usually mean and how to fix authentication and reputation. | Fix it | |
| GMX / WEB.DE "554 Transaction failed / Reject due to policy restrictions" and "invalid DNS PTR resource record": fix | GMX and WEB.DE refuse servers without a valid reverse DNS (PTR) or with a poor reputation. How to read the error and fix your server. | Fix it | |
| "554 5.7.1 Service unavailable; Client host [IP] blocked using zen.spamhaus.org": what it means and how to fix it | The receiving server found your IP on Spamhaus ZEN (SBL, XBL or PBL). How to find which list, fix the cause and get removed. SpamCop too. | Fix it | |
| "450 4.7.1 Client host rejected: cannot find your hostname" (or "reverse hostname"): how to fix it | The receiving server could not match your IP to a hostname. Fix the reverse DNS (PTR) and its matching A record to stop this bounce. | Fix it | |
| "530 5.7.0 Must issue a STARTTLS command first" (and "451 5.7.3 STARTTLS is required"): how to fix it | The server requires an encrypted connection before it accepts your email. Turn on TLS in your app, printer or server and use the right port. | Fix it | |
| "550 5.7.23 The message was rejected because of Sender Policy Framework violation" (SPF fail): how to fix it | The receiver rejected your email because your SPF record does not authorize the server that sent it. Find the server and update SPF. | Fix it | |
| "450 4.1.8 Sender address rejected: Domain not found" / "553 5.1.8 Domain of sender address does not exist": fix | The receiver could not find your sender domain in DNS. Usually a typo, a local domain like .local, or a domain with no A or MX record. | Fix it | |
| Mimecast "550 Rejected by header based Anti-Spoofing policy", "Envelope blocked" and Proofpoint "554 5.7.0 Blocked": fixes | Company email filters like Mimecast and Proofpoint blocked your email. What their bounces mean, what you can fix and how to ask the recipient's IT team. | Fix it | |
| "4.4.7 Message expired" and "Delivery Status Notification (Delay)" ("451 4.4.0 DNS query failed"): causes and fixes | Your server tried to deliver for hours or days, then gave up. Why emails get delayed and expire, and how to find and fix the cause. | Fix it |
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