SMTP stands for Simple Mail Transfer Protocol and is commonly used by most email servers. Your message will be transferred from one point to another, from your mailbox to the ones of your recipients. During these various “hops” (think of them as jumps), the servers will send back responses to indicate success or failure, as well as the reasons if the transmission fails. Here are some examples of SMTP responses, from the SendGrid site (https://sendgrid.com/blog/smtp-server-response-codes-explained):
250 – This SMTP server response simply means that everything went well, and your message was delivered to the recipient’s server (it does not mean that your message was opened…).
421 – Your message has been temporarily delayed by the recipient server. This is usually due to too many connections in a short period of time or too many messages.
450 – Your message was not delivered because the other user’s mailbox was not available. This can happen if the mailbox is locked or not routable.
451 – This response is sent when the message simply failed. Often, this is not your fault, but the fault of a remote server.
452 – This type of response is sent when there is not enough system storage to send the message. Your message is postponed until storage becomes available and it can be delivered.
550 – The message failed because the recipient’s mailbox is unavailable or because the receiving server rejected your message.
551 – The mailbox for which your message was intended does not exist on the recipient’s server.
552 – The mailbox to which your message was sent does not have enough storage space to accept your message.
553 – Your message was not delivered because the name of the sent mailbox does not exist.