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Troubleshooting & errors

Bounce codes, sync failures, and mail that won't arrive.

SMTP error 550 5.7.57 client was not authenticated to send anonymous mail during MAIL FROM, rejected by Microsoft 365 at the submission endpoint
Troubleshooting & errors

550 5.7.57 Client Not Authenticated During MAIL FROM: Fix

Fix 550 5.7.57 client was not authenticated to send anonymous mail during MAIL FROM — what it means, the causes, and the three Microsoft 365 sending paths.

Nafiul HasanNafiul Hasan
Illustration of an oversized email attachment blocked at a server size-limit checkpoint, with the file icon larger than the allowed threshold
Troubleshooting & errors

Attachment Too Large to Send: Limits and What to Do

Attachment too large to send email? Base64 encoding adds 33% overhead, so your true limit is below what your server states. Here is how to fix it.

Nafiul HasanNafiul Hasan
Illustration of duplicate email messages in an IMAP inbox with a header panel open showing the Message-ID field used to identify true duplicates
Troubleshooting & errors

Duplicate Emails in Inbox: Why IMAP Doubles Messages

Duplicate emails in an IMAP inbox come from four fixable causes. Here is how to tell which, remove the extra copy safely, and stop it recurring.

Nafiul HasanNafiul Hasan
Email app keeps asking for password — illustration of a credential prompt loop cycling between a mail client and an authentication server
Troubleshooting & errors

Email App Keeps Asking for Password: Why It Loops

Your email app keeps asking for password because a cached credential has gone stale. Here is how to find and remove it to stop the loop.

Nafiul HasanNafiul Hasan
Diagnostic illustration of an email sent but not received, tracing the delivery path through spam filters, quarantine, and authentication failures
Troubleshooting & errors

Email Sent but Not Received: How to Prove Where It Went

Email sent but not received? No bounce means the server accepted it, then filtered it. Here is how to trace exactly where it went and fix the cause.

Nafiul HasanNafiul Hasan
Diagram of the 550 5.7.1 email error showing a message being permanently refused by a receiving mail server on policy grounds
Troubleshooting & errors

550 5.7.1 Email Error: What It Means and How to Fix It

The 550 5.7.1 email error is a permanent policy or authentication rejection. Learn what it means, how to read the bounce text, and how to fix each cause.

Nafiul HasanNafiul Hasan
Bounce notification showing a 550 5.1.1 user unknown SMTP error with an envelope returned to sender — permanent recipient address failure explained
Troubleshooting & errors

550 5.1.1 User Unknown: What It Means and What to Do

550 5.1.1 user unknown means the mailbox doesn't exist. Here are the four causes and how to fix each one.

Nafiul HasanNafiul Hasan
Bounce notification showing a 550 5.4.1 recipient address rejected access denied SMTP error from Microsoft 365 Exchange Online — Directory Based Edge Blocking explained
Troubleshooting & errors

550 5.4.1 Recipient Address Rejected: Access Denied Fix

A 550 5.4.1 bounce from Microsoft 365 is almost always Directory Based Edge Blocking. Here are the three causes and whose admin needs to fix each.

Nafiul HasanNafiul Hasan
Diagram showing an SMTP server returning a 552 5.2.2 mailbox full permanent rejection, with a storage quota exceeded indicator blocking the incoming message
Troubleshooting & errors

552 5.2.2 Mailbox Full: What the Sender and Recipient Do

552 5.2.2 mailbox full is a permanent bounce — the sending server will not retry. Here is what the sender and recipient each do to fix it.

Nafiul HasanNafiul Hasan
SMTP error 535 5.7.8 username and password not accepted — an authentication rejection at the server boundary with causes and provider-specific fixes for Gmail and Microsoft 365
Troubleshooting & errors

535 5.7.8 Username and Password Not Accepted: The Fix

Fix SMTP error 535 5.7.8 username and password not accepted on Gmail, Microsoft 365 and generic IMAP hosts — causes table and step-by-step fixes.

Nafiul HasanNafiul Hasan
Diagram of a 550 5.7.26 unauthenticated email DMARC bounce, showing SPF and DKIM alignment both failing against the From domain
Troubleshooting & errors

550 5.7.26 DMARC Policy Rejection: How to Fix It

550 5.7.26 means unauthenticated email hit your domain's DMARC policy. Learn why SPF pass isn't SPF alignment, and fix it per sending service.

Nafiul HasanNafiul Hasan
Illustrated diagram of Gmail's incoming mail delivery path — storage check, filter rules, forwarding, and spam — with the eight causes of Gmail not receiving emails labeled at each stage
Troubleshooting & errors

Gmail Not Receiving Emails but Can Send: 8 Real Causes

Gmail not receiving emails but can send? Eight causes — filters, storage, forwarding, POP, blocked senders — and the fix for each.

Nafiul HasanNafiul Hasan

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