SMTP stands for Simple Mail Transfer Protocol. Your message will be relayed from server to server, until it makes it to the last link in the chain: the email server handling your recipient mailbox. This response is essentially an answer signalling what your recipient’ server has done with the email you’ve sent. A few examples of response codes from SendGrid’s website (https://sendgrid.com/blog/smtp-server-response-codes-explained) :
250 – This SMTP server response simply means everything went well and your message was delivered to the recipient server (this does not mean your message was opened
421 – Your message was temporarily deferred by the recipient server. This is usually a result of too many connections in a short timeframe or too many messages.
450 – Your message was not delivered because the other user mailbox was not available. This can happen if the mailbox is locked or is not routable.
451 – This response is sent when the message simply failed. Often times this is not caused by you, but rather because of a far-end server problem.
452 – This kind of response is sent back when there isn’t enough system storage to send the message. Your message is deferred until storage opens up and it can then be delivered.
550 – The message has failed because the other user’s mailbox is unavailable or because the recipient server rejected your message.
551 – The mailbox your message was intended for does not exist on the recipient server.
552 – The mailbox your message was sent to does not have enough storage to accept your message.
553 – You message was not delivered because the name of the mailbox you sent to does not exist.