Os bounces têm um código numérico que ajuda a identificar o motivo da falha de entrega. Aqui está a descrição técnica dos mais frequentes, separados por categorias:

Endereço de email inválido ou inexistente

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


Caixa cheia

5.2.2    Mailbox full
5.3.1    Mail system full


Mensagem rejeitada (devido ao conteúdo, tamanho ou outro elemento)

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


Problemas diversos no próprio sistema de correio de destino

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