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The Computer Corner


By Charles Miller


A Message Demon


“Why didn’t my email go through?” This is a question that many people wonder about often when they get that disappointing error message telling them that their email wasn’t delivered, along with a code involving a set of mysterious-looking numbers punctuated by dashes and dots.

The protocol for sending emails is called Simple Mail Transport Protocol (SMTP), and when it fails, there could be hundreds of possible reasons why that happened. Many times when an email was not transmitted successfully, the numbers in these SMTP error codes can be very helpful for diagnosing and correcting the problem.

The bounced message appearing in your inbox usually comes from “” and often has a subject line reading “Message Delivery Failure.” The message may say “Delivery to the following recipients failed permanently,” and below that may be found the SMTP error message number.

Here’s some examples:

Error number 550-5.1.1 means the email address you entered does not exist. Check your spelling of the name.

Error number 553-5.1.2 means the domain name is wrong. Check to be sure you did not misspell as, for example.

Error number 421-4.7.0 happens when an unusually high number of emails originate from your IP address. Expect to see that one if you are using public Wi-Fi and there are a dozen other patrons at the coffee shop all sending emails.

Error number 452-4.2.2 usually means the recipient’s inbox is full and cannot accept any more emails.

Error number 552-5.2.2 means the email recipient has gone over a daily limit of how many messages they are permitted to receive in one day.

Error number 452-4.5.3 indicates that the email you tried to send has too many recipients. You can expect to be identified as a spammer when you try sending one email to too many people at once.

Error number 535-5.7.1 means your username or password—usually the one saved on your device—was not recognized.

Error number 530-5.5.1 usually means the security settings on your computer are not configured correctly.

Error number 550-5.4.5 usually means you have sent too many emails today and cannot send any more until you wait a while.

Error number 550-5.7.1 means the receiving server tagged your mail as being unsolicited spam and blocked it.

Error number 552-5.2.3 means you are trying to send an email attachment that is too big. Most email servers have a size limit for attachments, and unfortunately, the limit is set by company policy and differs from company to company.

Error number 421-4.4.5 or 450-4.2.1 means nothing is wrong at all except that a mail server is just too busy right now. Try sending your email again later.

As you can see, there could be many different reasons why your email did not go through to its destination. There are more SMTP error codes than listed here—many, many more. If you received a bounced email, you should do a web search of the code sent back to you with the error message to find more information pertaining to that specific error number.

Charles Miller is a freelance computer consultant, a frequent visitor to San Miguel since 1981, and now practically a full-time resident. He may be contacted at 044 415 101 8528 or email FAQ8 (at)


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