Knowledge Base

550 Blocked Error Explained

550 Blocked error or 550 Requested action not taken: mailbox unavailable is an SMTP (Simple Mail Transfer Protocol) error code. Put simply, this message means that the email you sent was blocked by the recipient's email hosting server, and returned to you.

NOTE: This error is often a problem at the recipient’s end, meaning it is probably not caused by your own email hosting server.


SMTP (Simple Mail Transfer Protocol) is a set of standardized rules on sending and receiving emails (electronic mails). Most often, it is used along with POP3 (Post Office Protocol 3) or IMAP (Internet Message Access Protocol) because it has limitations with regards to queueing and receiving messages. Hence, the usual setup for emails is to use SMTP for outgoing emails while it is POP3 or IMAP for incoming emails.

This SMTP 550 error code can be written in several different ways. Usually, the phrasing of the error message depends on the hosting server sending the error. Here are some common ways they may be phrased:

  • SMTP Error 550
  • SMTP Error 5.5.0
  • 550 Invalid recipient
  • 550 User account is unavailable
  • 550 – Requested actions not taken as the mailbox is unavailable.
  • 550 Address rejected
  • 550 No such user here
  • 550 Not our Customer
  • 550 Account not available
  • Remote host said: 550 – [email protected], this MYDOMAIN.COM
    Mailbox Does Not Exist – Giving up
  • does not like recipient
    Remote host said: 550-Verification failed for [email protected]
    550-Previous (cached) callout verification failure
    550 Sender verify failed Giving up on
  • The attached file had the following undeliverable recipient(s): [email protected]

    Transcript of the session follows:
    Command: Data...
    Response: 550 Cyberoam Anti Spam Engine has identified this email as spam. Refid:str=0001.0A0B0206.4BA7422C.01F5,ss=1,vtr=str,vl=0,pt =
  • 550-Verification failed for [email protected] - 550-Unrouteable address - 550 Sender verify failed (in reply to RCPT TO command)
NOTE: If there is a system-wide problem with the recipient's hosting server or the host is temporarily down, the following error message is also possible:
  • 5.1.0 - Unknown address error 550-'Mailbox is inactive'

What Are the Causes of a 550 Blocked Error?

The 550 SMTP error is one of the most common error codes you may encounter. For this particular error code, here are some of the possible causes:

  1. Your IP address is blocked by the recipient’s email hosting server.
  2. Your email address has been compromised and has been blocked by the recipient’s hosting server.
  3. You are sending an email using an ISP that is different to the one you are currently logged on to.
  4. The recipient’s email address cannot be found, or does not exist.
  5. The recipient’s email has been suspended or temporarily disabled.
  6. Your own domain name is invalid, cannot be found, or has been blocked.
  7. Your email message is being treated as spoofed, and has been rejected by the recipient’s email hosting server.
  8. The recipient’s email hosting server tried to verify the sender's email address, but was unable.
  9. The recipient’s hosting server is down, and is unable to receive the email.
  10. The recipient’s inbox is full, and is unable to receive the email.
  11. The recipient’s email hosting has an anti-spam firewall, and has labelled your message as spam.
  12. Your own Internet gateway has a firewall, or filtering feature, that intercepted your own email, regarded it as spam, and blocked it to stop you from accidentally sending out spam or a virus-infected email.
  13. Your email account has been hacked, has been used to send out tons of spam mail, and so has been blocked by the recipient’s SPF (Sender Policy Framework).

SMTP 550 Error is often a problem at the recipient’s end, which means this error is usually caused by factors outside your email hosting server.


Easy Ways To Troubleshoot a 550 Blocked Error

Although the SMTP 550 Error is usually an error on the recipient’s end, there may still be ways you can fix the problem. Here are some possible solutions:

  1. Double-check the recipient’s email address. Make sure you have the right email address and that there are no misspellings, typos, or extra spaces on it.

  2. Confirm with the recipient that you have their correct email address. If you have checked the email address and you are certain you typed it correctly, but you still get an SMTP 550 Error, then perhaps you got the wrong email address from the recipient. It could also be that they have changed the domain name or any other part of the address.

  3. Check your own email address and message. Make sure that your sender email is typed correctly, and check that your email has content. Some email hosting servers can reject emails that are blank, or have no content.

  4. Try changing your outgoing mail port to 26 or 465. Some servers block port 25 because spammers often use it to send spam, or spoofed emails.

  5. Scan your computer for viruses and malware. Technically, email addresses don’t get infected; it’s the computers or the hard drives that do. Install a reliable antivirus program, and scan your computer. After the scan, change your email password, just in case you have been hacked.

  6. Re-try sending the email after a few minutes, or hours. It is possible that the recipient’s server is just temporarily down and may come back up after a while. If you get the error message again, even after a few hours, contact the recipient and inform them that their hosting may be down.

  7. Use your ISP’s outgoing mail server. Sometimes, it is your own Internet Service Provider or Internet gateway filtering and blocking your emails. If this is the case, you can try using your ISP’s outgoing mail server. You can check our partial list of outgoing mail servers for some popular ISPs here .

  8. Contact your ISP. A good solution may be to ask your Internet Service Provider for assistance. After all, they are the ones who know their system best, and may be able to help.

  9. Change all your passwords. Be sure to use a random mix of letters, numbers, and special characters to make a really strong password. Also change your recovery options, since if hackers have gained access to your account they would probably be aware of these options, too. As added protection, make a habit of changing your passwords regularly.
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