Understanding Bounce Messages in Email

• What Are Bounce Messages?

  When you send an email, it travels through mail servers before reaching the recipient. If a server cannot deliver your email, it will send back a bounce message to let you know what went wrong.

  Think of it as the server saying:
  ❌ “We couldn’t deliver this email, and here’s why.”

• Types of Bounce Messages

  Hard Bounce (Permanent failure) The email will never be delivered. Example causes: The email address doesn’t exist. The domain is invalid.

  Soft Bounce (Temporary failure) The delivery failed, but it may work later. Example causes: The recipient’s mailbox is full. The server is temporarily unavailable.

  Common Examples

• Address or Domain Issues

  Error Message	What It Means	What You Can Do
  550 5.1.1 User unknown	The email address doesn’t exist.	Check for typos or confirm the recipient’s email.
  550 5.1.2 Host unknown	The domain doesn’t exist.	Ensure the domain (e.g. example.com) is correct.
  550 5.1.0 Address rejected	The server doesn’t recognise the address.	Confirm the recipient’s address is valid.

• Mailbox Problems

  Error Message	What It Means	What You Can Do
  421 4.2.1 Mailbox full	The recipient’s inbox is full.	Wait and try again later.
  450 4.2.0 Mailbox unavailable	The mailbox is temporarily disabled or blocked.	Try later or contact the recipient.
  452 4.2.2 Insufficient system storage	The recipient’s mailbox or system ran out of space.	Wait until the issue is resolved.

• Server or Connection Issues

  Error Message	What It Means	What You Can Do
  450 4.2.0 Greylisting	The server is temporarily rejecting the email.	Usually clears up; try again later.
  451 4.4.1 Connection timed out	The recipient’s server isn’t responding.	Retry later; persistent errors mean the server may be down.
  421 4.4.2 Connection dropped	The connection was unexpectedly closed.	Retry later.
  421 4.7.0 Temporary server error	A temporary issue on the recipient’s server.	Wait and try again later.

• Message Size & Content Issues

  Error Message	What It Means	What You Can Do
  552 5.2.3 Message size exceeds fixed limit	Your email (including attachments) is too large.	Reduce attachment size and resend.
  552 5.3.4 Message size limit exceeded	The recipient’s system has stricter size limits.	Compress attachments or use file-sharing services.
  554 5.6.0 Invalid message content	The message format is not accepted (e.g. corrupted headers).	Check formatting or resend with plain text.

• Security & Policy Blocks

  Error Message	What It Means	What You Can Do
  550 5.7.1 Relaying denied	The server won’t pass on the message.	Check outgoing server settings or authentication.
  550 5.7.1 Message rejected as spam	The server flagged your message as spam.	Reword the message or remove suspicious attachments.
  550 5.7.25 SPF validation failed	Sender Policy Framework (SPF) check failed.	[Domain Mailbox] Ask your email admin to review DNS SPF settings.
  550 5.7.26 DKIM signature invalid	DomainKeys Identified Mail (DKIM) check failed.	[Domain Mailbox] Ask your email admin to fix DKIM records.
  550 5.7.27 DMARC policy failure	Domain’s DMARC policy rejected the mail.	[Domain Mailbox] Contact your domain/email admin to review DMARC setup.

• Why This Matters

  Bounce messages help you:

  • Understand if the problem is with the email address or the recipient’s server.
  • See whether the error is temporary or permanent.
  • Decide on the next step: retry, correct the address, or reach out another way.