What is an SMTP 550 Error?

An SMTP 550 error message or '550 Requested action not taken: mailbox unavailable error' message is an SMTP (Simple Mail Transfer Protocol) error. This message is displayed when the email sent by a sender has been blocked by the recipient's email hosting server, and returned to the sender.

Note: This error is mostly caused by a problem at the recipient’s end, not the email hosting server of the email sender.

SMTP (Simple Mail Tranfer Protocol) is a set of standardised rules for sending and receiving emails (electronic mail). It is used alongside POP3 (Post Office Protocol 3) or IMAP (Internet Message Access Protocol) because it has limitations regarding the queueing and receiving of messages. Hence, email accounts are typically set up to use SMTP for outgoing emails while incoming email is managed using POP3 or IMAP.

The SMTP 550 error code can be displayed in various forms. Usually, the phrasing of the error message depends on the hosting server sending the error. Here are some common ways SMTP 550 error codes may be phrased:

  • SMTP Error 550
  • SMTP Error 5.5.0
  • 550 Blocked
  • 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 – user@mydomain.com, this MYDOMAIN.COM; Mailbox Does Not Exist – Giving up
  • does not like recipient; Remote host said: 550-Verification failed for user@mydomain.com; 550-Previous (cached) callout verification failure; 550 Sender verify failed Giving up on

Note: If there is a system-wide problem with the recipient's hosting server or the host is temporarily down, the following error message may also present:

  • 5.1.0 - Unknown address error 550-'Mailbox is inactive'


What are the causes of an SMTP 550 error?

The 550 SMTP error is quite a common error encountered when troubleshooting email transmission issues. Here are some of the possible causes of a 550 SMTP error code:

  • Your IP address is blocked by the recipient’s email hosting server.
  • You are attempting to send an email using an ISP that is different to the one you are currently logged into.
  • Your email address has been blocked by recipient’s hosting server.
  • The recipient’s email address cannot be found or does not exist.
  • The recipient’s email account has been disabled or suspended.
  • Your email message is being treated as spam, and has been rejected by the recipient’s email hosting server.
  • The recipient’s email hosting server was unable to verify your email address.
  • The recipient’s hosting server is down, and is unable to receive emails.
  • The recipient’s inbox has reached its quota, and is unable to receive anymore emails.
  • Your own Internet gateway firewall your email as spam and intercepted it, to stop it being accidentally sent out spam or a virus-infected email.
  • Your email account has been hacked and used to send out a large number of spam emails, and these have been blocked by the recipient’s SPF (Sender Policy Framework).


Troubleshooting tips to fix an SMTP 550 error

A SMTP 550 Error is typically an error at the recipient’s end. However, there are several things you can do to fix the problem:

    1. Double-check the recipient’s email address. Make sure that the email address has been entered correctly and that there is no incorrect spelling, typographic errors or punctuation errors in it.
    2. Confirm the email address with the recipient. If you have checked that you have entered email address correctly, but you still receive an SMTP 550 Error, then the recipient may have given you the wrong email address. Their email address may have changed as part of their business processes or an email infrastructure change (such as a domain name change).
    3. Check that your email address is entered correctly, and that your email does not contain spam content. Some email hosting servers will reject emails that are blank, or have content that looks like spam. Keywords in the email subject field like “Test”, or “Spam” are more likely to cause your email to be filtered out by the recipient’s email hosting server.
    4. Try changing your outgoing mail port to 465 or 587. Some servers block port 25 because it does not require SSL (Secure Sockets Layer) or TLS (Transport Layer Security) and is easily used by spammers to send spam, or spoofed emails.
    5. Wait a few hours then resend your email. It is possible that the recipient’s server is temporarily offline, or being restarted and may come back online after a while. If you get the error message again, even after a few hours, contact the recipient and inform them that their email hosting server may be down.
    6. 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 settings.
    7. Contact your ISP for assistance. It might be a good idea to ask your Internet Service Provider for assistance. They may be aware of a issue affecting all their clients and have already found a good solution.
    8. Change your email account passwords. Ensure you use a random mix of letters (both capitalized and lowercase), numbers, and special characters to increase the strength of your password. Also, check your email account recovery options; if hackers have gained access to your account they will probably be aware of your recovery options and may change these to point towards their own accounts. Get into a good habit of changing your passwords regularly.
    9. Scan your computer for viruses and malware. Install a reputable antivirus program like Bitdefender® , and scan your computer. After the scan, change the password to your email account Incase you have been hacked.
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