Why Emails Bounce (Common SMTP Errors Explained)

Email bounces occur when a message cannot be delivered to the recipient. Bounce messages usually include an SMTP error code, which explains the reason.

Common SMTP Errors & What They Mean

550 – Mailbox Not Found

Cause

• The email address does not exist

• Typo in the recipient’s address

Fix

• Confirm the email address is correct

• Check that the mailbox exists in Eclipse Portal

552 – Mailbox Full

Cause

• Recipient’s inbox has exceeded its storage quota

Fix

• Ask the recipient to delete or archive old emails

• Increase mailbox storage if it’s your account

554 – Message Rejected / Spam Detected

Cause

• Email content triggered spam filters

• Missing SPF, DKIM, or DMARC records

Fix

• Avoid spam-like wording or large attachments

• Ensure domain email authentication is enabled

421 – Service Not Available

Cause

• Temporary server issue or rate limiting

Fix

• Wait a few minutes and resend

• Avoid sending bulk emails rapidly

530 – Authentication Required

Cause

• Outgoing mail server requires login credentials

Fix

• Ensure SMTP authentication is enabled in your email client

Where to Check Email Accounts

1. Log into Eclipse Portal

2. Open Email Accounts

3. Verify mailbox exists and storage limits are not exceeded