SMTP Error 550: Mailbox Unavailable (Permanent)

SMTP error 550 indicates that the requested mail action was not taken because the mailbox is permanently unavailable. This is a permanent failure that typically occurs when the mailbox doesn't exist, has been deleted, or is permanently disabled.

What Does 550 Mean?

The SMTP reply code 550 follows the standard SMTP response format:

• 5 = Permanent failure (permanent negative completion reply)
• 50 = Requested mail action not taken: mailbox unavailable

When you receive a 550 error, it means the receiving mail server cannot deliver to the recipient's mailbox because it doesn't exist or is permanently unavailable. This is a hard bounce that should result in removing the address from your mailing list.

Error Type

• Type: Permanent failure (hard error)
• Category: Mailbox availability
• Action Required: Remove the email address from your mailing list

Common Causes

1. Mailbox Doesn't Exist: The email address doesn't exist on the mail server
2. Mailbox Deleted: The mailbox has been permanently deleted
3. Account Disabled: The email account has been permanently disabled
4. Domain Doesn't Exist: The domain portion of the email address doesn't exist
5. Invalid Address: The email address format is invalid or malformed
6. Account Suspended: The account has been permanently suspended
7. Server Configuration: Server configuration prevents mailbox access

How to Resolve

For Email Marketers

1. Remove from List: Immediately remove the email address from your mailing list
2. Verify Address: Double-check the email address for typos or errors
3. Use Double Opt-in: Implement double opt-in to ensure email addresses are valid
4. Clean Your List: Periodically clean your email list to remove invalid addresses
5. Monitor Bounce Rates: Keep your hard bounce rate below 2% to maintain good deliverability

For Developers

1. Remove from Database: Remove the email address from your database immediately
2. Implement Bounce Handling: Set up automated bounce processing to remove hard bounces
3. Use Email Verification: Consider using email verification APIs to check addresses before sending
4. Track Bounce Codes: Log bounce codes in your system to identify patterns
5. Suppression Lists: Maintain suppression lists of bounced addresses

Related SMTP Errors

• 450 - Mailbox Unavailable (Temporary) - Temporary mailbox unavailability
• 551 - User Not Local - User not local to server
• 553 - Mailbox Name Not Allowed - Invalid mailbox name
• 554 - Transaction Failed - Transaction failures

Examples

Example Error Message

550 Requested mail action not taken: mailbox unavailable

Example with Enhanced Status Code

550 5.1.1 User unknown

Common Email Provider Responses

• Gmail: "550 The email account that you tried to reach does not exist"
• Outlook/Hotmail: "550 5.1.1 User unknown"
• Yahoo: "550 Invalid recipient"
• Custom Domains: Varies by mail server configuration

Best Practices

1. Never Retry: Don't attempt to resend emails to addresses that bounce with 550
2. Remove Immediately: Remove bounced addresses from your mailing list immediately
3. Maintain List Hygiene: Regularly clean your email list to remove invalid addresses
4. Monitor Metrics: Track your hard bounce rate and aim to keep it below 2%
5. Use Suppression Lists: Maintain suppression lists of bounced addresses
6. Implement Real-time Validation: Validate email addresses at the point of collection
7. Track Patterns: Monitor 550 errors to identify data quality issues
8. User Communication: Notify users when their email addresses bounce