SMTP 550 Errors Explained: What They Mean and How to Fix Them

• 2Fifteen Tech
Email Security DMARC

SMTP error 550 means the recipient’s mail server rejected your email outright. Unlike temporary failures that resolve on their own, a 550 is permanent—the server won’t retry delivery, and the message isn’t going through until the underlying issue is fixed. The problem is that “550” covers a wide range of causes, and the fix depends entirely on which variant you’re dealing with.

How to Read a 550 Error

Most 550 errors include an enhanced status code in the format 550 X.Y.Z followed by a human-readable message. The three-digit enhanced code breaks down like this:

  • First digit (5): Permanent failure—the server won’t retry
  • Second digit: The category of the problem (address, mailbox, system, etc.)
  • Third digit: The specific issue within that category

For example, 550 5.1.1 means permanent failure (5), address-related issue (1), bad destination mailbox (1). Understanding this structure helps you diagnose the problem faster, even when the accompanying message is vague.

The Most Common 550 Errors

550 5.1.1 — User Unknown / Recipient Does Not Exist

What it means: The email address you sent to doesn’t exist on the recipient’s server.

Common causes:

  • Typo in the email address
  • The recipient’s account was deleted or deactivated
  • The domain is valid but the specific mailbox isn’t

How to fix it: Double-check the address for typos. If you’re sure it’s correct, the recipient may have changed email addresses or left the organization. Reach out through another channel to confirm.

550 5.1.10 — Recipient Not in Directory

What it means: Similar to 5.1.1, but specifically indicates the address isn’t listed in the recipient server’s directory.

Common causes:

  • Sending to an address that was never created
  • Directory sync issues on the recipient’s Exchange or Microsoft 365 environment

How to fix it: Verify the address with the recipient. If it’s a corporate email, their IT team may need to check directory sync or ensure the mailbox is properly provisioned.

550 5.2.1 — Mailbox Cannot Receive Email

What it means: The mailbox exists but can’t accept messages right now.

Common causes:

  • The mailbox is full (over quota)
  • The account has been disabled by an administrator
  • Mailbox settings are blocking external email

How to fix it: This one’s on the recipient’s side. If you can reach them another way, let them know their mailbox isn’t accepting email. They’ll need to free up space or have their admin re-enable the account.

550 5.4.5 — Daily Sending Quota Exceeded

What it means: You’ve hit your email provider’s sending limit for the day.

Common causes:

  • Sending too many emails in a 24-hour period
  • A compromised account sending spam without your knowledge
  • Using a personal email plan for business-volume sending

How to fix it: Wait for the limit to reset (usually 24 hours). If this is a recurring issue, upgrade to a plan with higher limits or use a dedicated email sending service for bulk communications. Also check for signs of account compromise—if you didn’t send those emails, someone else did.

550 5.7.0 — Message Size Violation

What it means: Your email exceeds the maximum message size the recipient’s server allows.

Common causes:

  • Large file attachments
  • Embedded images inflating the message size
  • The recipient’s server has a lower size limit than your provider

How to fix it: Compress attachments, use a file-sharing service (Google Drive, Dropbox, etc.) for large files, or split the content across multiple emails. Most servers cap messages at 25-50MB, but some set much lower limits.

550 5.7.1 — Message Rejected (Policy or Permissions)

What it means: This is the catch-all rejection code. The recipient’s server blocked your message based on its security policies.

Common causes:

  • Your IP address or domain is on a blocklist
  • The message content triggered spam filters
  • Your server isn’t authorized to relay through the recipient’s server
  • The recipient has content restrictions blocking your message
  • Failed authentication checks

How to fix it: This one requires some detective work since the same code covers multiple scenarios. Start by checking whether your domain or IP is on any blocklists using a tool like MXToolbox. Review your email content for spam triggers—excessive links, suspicious attachments, or all-caps subject lines. Verify your SMTP authentication settings are correct.

550 5.7.15 — SPF Check Failed

What it means: The recipient’s server checked your domain’s SPF record and determined that the server sending your email isn’t authorized.

Common causes:

  • Your SPF record doesn’t include the sending server
  • You recently changed email providers but didn’t update SPF
  • You exceeded the 10-DNS-lookup limit in your SPF record
  • Your SPF record has syntax errors

How to fix it: Check your SPF record to make sure it includes every service that sends email on behalf of your domain. Watch the 10-DNS-lookup limit—each include: and redirect counts toward it, and exceeding it causes SPF to fail silently.

550 5.7.26 — Email Authentication Failed

What it means: The recipient’s server (typically Gmail) couldn’t verify your email against SPF, DKIM, and DMARC.

Common causes:

  • Missing or misconfigured SPF, DKIM, or DMARC records
  • DMARC alignment failure (the “From” domain doesn’t match SPF or DKIM domains)
  • Third-party services sending email as your domain without proper authentication

How to fix it: We wrote a detailed guide on this specific error: 550 5.7.26 Gmail Error: What It Means and How to Fix It. The short version: verify your SPF, enable DKIM, publish a DMARC record, and make sure every service sending email as your domain is properly authenticated.

550 Relay Not Permitted

What it means: The mail server you’re trying to send through won’t relay your message to the recipient.

Common causes:

  • SMTP authentication isn’t enabled in your email client
  • Incorrect outgoing server credentials
  • Your email client is configured to use the wrong SMTP server

How to fix it: Check your outgoing mail server settings. Make sure SMTP authentication is enabled and your credentials are correct. If you recently changed networks or VPNs, your client may need to re-authenticate.

550 High Probability of Spam / Message Classified as Spam

What it means: The recipient’s server flagged your email as spam and refused delivery.

Common causes:

  • Your domain or IP has a poor sending reputation
  • Email content matches spam patterns
  • Missing authentication records (SPF, DKIM, DMARC)
  • Your domain is on a real-time blocklist (RBL)
  • High spam complaint rate from previous sends

How to fix it: Start with authentication—make sure SPF, DKIM, and DMARC are all properly configured. Check your domain reputation using Google Postmaster Tools. Review your email content for spam triggers. If your IP is blocklisted, follow the removal process for the specific blocklist. For more background on email authentication, see our guide: Your Guide to Email Security and Deliverability.

When It’s Not Your Problem to Fix

Some 550 errors point to issues on the recipient’s side that you can’t resolve yourself:

  • Mailbox full or disabled — The recipient needs to clear space or contact their admin
  • User doesn’t exist — The address was deleted or never created
  • Recipient directory issues — Their IT team needs to fix directory sync

In these cases, the best you can do is contact the recipient through another channel and let them know there’s an issue with their email.

When It Is Your Problem

If you’re seeing 550 errors consistently across multiple recipients, the issue is almost certainly on your end. The most common culprits:

  1. Missing email authentication — Set up SPF, DKIM, and DMARC if you haven’t already
  2. Blocklisted domain or IP — Check and request removal from any blocklists
  3. Poor sender reputation — Monitor with Google Postmaster Tools and reduce spam complaints
  4. Incorrect SMTP settings — Verify your outgoing server configuration
  5. Sending limits exceeded — Stay within your provider’s daily limits

How We Can Help

At 2Fifteen Tech, we configure and manage email authentication for businesses so bounce-backs and deliverability issues don’t land on your desk. We’ll audit your DNS records, fix SPF/DKIM/DMARC misconfigurations, and set up monitoring so problems get caught before they affect your business communications.

Our fully managed DMARC service handles the ongoing monitoring and reporting that keeps your email flowing and your domain protected.

If 550 errors are disrupting your business email, we can help sort it out.

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