What do all those email bounce codes mean?

A bounce is the automated reply you get everytime an email address can't accept the email you send it. Usually this happens when the email address is invalid or has been disabled, but there are many other possible reasons.

Bounces include a numeric code which helps understand why your email was not delivered. Find below a description for most standard codes, sorted by categories: 

Invalid or expired email address

421      Service not available, closing transmission channel
450      Requested mail action not taken: mailbox unavailable (e.g., mailbox busy)
451      Requested action aborted: error in processing
550      User’s mailbox was unavailable (such as not found)
551      The recipient is not local to the server.
553      The command was aborted because the mailbox name is invalid.
5.0.0    Address does not exist
5.1.1    Bad destination mailbox address
5.1.2    Bad destination system address
5.1.3    Bad destination mailbox address syntax
5.1.4    Destination mailbox address ambiguous
5.1.5    Destination mailbox address valid
5.1.6    Mailbox has moved
5.1.7    Bad sender’s mailbox address syntax
5.1.8    Bad sender’s system address
5.2.0    Other or undefined mailbox status
5.2.1    Mailbox disabled, not accepting messages
5.1.0    Other address status
5.3.0    Other or undefined mail system status
5.4.1    No answer from host
5.4.2    Bad connection
5.4.0    Other or undefined network or routing status
5.4.3    Routing server failure
5.4.4    Unable to route
5.4.7    Delivery time expired
5.5.0    Other or undefined protocol status

Mailbox full

5.2.2    Mailbox full
5.3.1    Mail system full

Message rejected (due to contents, size or something else)

5.2.3    Message length exceeds administrative limit.
5.2.4    Mailing list expansion problem
5.3.4    Message too big for system
5.5.3    Too many recipients
5.7.0    Other or undefined security status
5.7.1    Delivery not authorized, message refused
5.7.2    Mailing list expansion prohibited
5.7.7    Message integrity failure

Miscellaneous issues in the mail server you sent the message to

452      Requested action not taken: insufficient system storage
500      The server could not recognize the command due to a syntax error.
501      A syntax error was encountered in command arguments.
502      This command is not implemented.
503      The server has encountered a bad sequence of commands.
504      A command parameter is not implemented.
552      The action was aborted due to exceeded storage allocation.
554      The transaction failed for some unstated reason.
5.3.2    System not accepting network messages
5.3.3    System not capable of selected features
5.4.5    Network congestion
5.4.6    Routing loop detected
5.5.1    Invalid command
5.5.2    Syntax error
5.5.4    Invalid command arguments
5.5.5    Wrong protocol version
5.6.0    Other or undefined media error
5.6.1    Media not supported
5.6.2    Conversion required and prohibited
5.6.3    Conversion required but not supported
5.6.4    Conversion with loss performed
5.6.5    Conversion failed
5.7.3    Security conversion required but not possible
5.7.4    Security features not supported
5.7.5    Cryptographic failure
5.7.6    Cryptographic algorithm not supported

Are there other bounce codes in addition to the above?

Yes. The above is just the standard set. Mail server administrators can customise it, so you're bound to come across different codes. For example, major ISPs have their own. Check them out:


Do bounces hurt my deliverability?

Definitely! That's why E-goi handles them automatically for you. Check out how it's done!