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Email Bounces: Why It Happens And How To Lower The Bounce Rate

Have you ever sent out an email to someone or a group of people? Sending out emails is supposed to be a button press’ work. However, a combination of factors may combine to hamper your email’s deliverability to the intended recipients. Email bounces have become commonplace in the corporate world. Bouncing emails are a marketer’s worst nightmare.

But why do emails bounce? Let’s get to understand this.

First, it is important to know that there are two types of bounces; Hard bounces and soft bounces.

A hard bounce happens when the email is permanently rejected because the email address is invalid or the email address doesn’t exist.

A soft bounce happens when the email address is valid and was delivered to the recipient’s inbox, but it still bounced because the recipient’s mailbox was full, the server was down, or the message was too large (more than 30 MB) for the recipient’s inbox.

Well then, let’s also understand the specific reasons for the hard and soft bounces.

Email address does not exist

When a non-existent email address is a reason for your email bouncing, the reason could be that the recipient gave you the wrong address or that the email has a typo. For hosted emails, it could be that the holder of the email address deleted it or left the company he was working in.

When this happens, it is important to review the email and find out if there are any obvious typing mistakes or reach out to the address holder for the correct email address.

Email address is blocked

The receiving server can also block incoming mail messages from a particular address.

The reasons for this range from spamming to sending mails from an infected device. Institutions within the financial sector, for example, have set up their servers to reject messages and block addresses that come from alarming addresses.

When your email is blocked, it is important to get in touch with server administrators who will analyze the reasons for why your address was blocked and advise accordingly.

Undeliverable email

An email being labelled ‘Undeliverable’ means the receiving mail server was temporarily unavailable at the time of sending, was offline or could not be found. When a server is under maintenance or has crashed, it ‘cannot be found.’

When this happens, you can wait for some time to send the mail message again. If it fails repeatedly, the reason could be that the server is permanently offline.

The recipient’s mailbox is full

If the recipient’s address has reached capacity, your email, and any other incoming mail messages, will bounce until there is space for them.

When this happens, you can get in touch with your recipient to create space for incoming mail messages or expand the message limit to accommodate more.

Email is blocked by antispam cloud

Email receivers employ complex antispam mechanisms that can sometimes affect legitimate and harmless emails. When your computer or network is compromised by spammers (mostly without your knowledge) mail receivers will temporarily block emails coming from your server for spamming. If this happens to your mails, you will receive an error message, assigned numbers 500 to 600 followed by a textual description of the problem.

SMTP Errors

An SMTP Server error is caused by an error in the setup in your email client program for the outgoing (SMTP) mail server. A mistake in the username, password, server address, or server authentication produces this error message when attempting to send an email. To correct this error, you need to know from your email provider what type of authentication, if any, is required by their mail server.

Most email bounces will have the following codes

  • 211 – System status/system help reply
  • 214 – Help message
  • 220 – Domain service ready
  • 221 – Domain service closing transmission channel
  • 250 – Requested mail action completed and OK
  • 251 – Not Local User, forward email to the forward path
  • 252 – Cannot Verify user, will attempt delivery later
  • 253 – Pending messages for node started
  • 354 – Start mail input; end with.
  • 355 – Octet-offset is the transaction offset
  • 421 – Domain service not available, closing transmission channel
  • 432 – Domain service not available, closing transmission channel
  • 450 – Requested mail action not taken: mailbox unavailable. request refused
  • 451 – Requested action aborted: local error in processing Request is unable to be processed, try again
  • 452 – Requested action not taken: insufficient system storage
  • 453 – No mail
  • 454 – TLS not available due to temporary reason. Encryption required for the requested authentication mechanism
  • 458 – Unable to queue messages for node
  • 459 – Node not allowed: reason
  • 500 – Syntax error, command unrecognized
  • 501 – Syntax error in parameters or arguments
  • 502 – Command not implemented
  • 503 – Bad sequence of commands
  • 504 – Command parameter not implemented
  • 510 – Check the recipient address
  • 512 – Domain can not be found. Unknown host.
  • 515 – Destination mailbox address invalid
  • 517 – Problem with senders mail attribute, check properties
  • 521 – Domain does not accept mail
  • 522 – Recipient has exceeded mailbox limit
  • 523 – Server limit exceeded. Message too large
  • 530 – Access Denied. Authentication required
  • 531 – Mail system Full
  • 533 – Remote server has insufficient disk space to hold email
  • 534 – The authentication mechanism is too weak. Message too big
  • 535 – Multiple servers using the same IP. Required Authentication
  • 538 – Encryption required for the requested authentication mechanism
  • 540 – Email address has no DNS Server
  • 541 – No response from host
  • 542 – Bad Connection
  • 543 – Routing server failure. No available route
  • 546 – Email looping
  • 547 – Delivery time-out
  • 550 – Requested action not taken: mailbox unavailable
  • 551 – User not local; please try forward path
  • 552 – Requested mail action aborted: exceeded storage allocation
  • 553 – Requested action not taken: mailbox name not allowed
  • 554 – Transaction failed

Misconfigured MX Records

MX Records are very key to sending and receiving mail as well as deliverability. When someone sends an email, the sending mail server will look up the MX record in DNS by following a sequence along these lines:

  1. Look up the authoritative name servers for the domain
  2. Query the domain name servers for the MX records
  3. Look up the names of the MX records in DNS to get their IP addresses

If the records are misconfigured, the server will simply reject the mail (the email will bounce)

For more on the importance of MX Records in mailing, read this.

Email bounce backs can happen any time. Whenever it happens, never hesitate to send us an email with a screenshot of the error message you get. Our support team will troubleshoot the error and suggest best practices to avoid a similar scenario. Our customer support email is;

The Author

Janet M.

Janet is a digital media specialist with 4+ years of experience in technical writing for the web hosting industry. She is also passionate about creative writing and storytelling. She combines her technical knowledge with her creativity to produce engaging content that informs and captivates readers across various platforms.

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