What is an SMTP response?

SMTP stands for Simple Mail Transfer Protocol, commonly used by the majority of email servers. Your message will be transferred from one point to another, from your inbox to that of your recipients. During these various "hops" (think of them as jumps), servers will return responses to indicate success or failure, along with the reasons if the transmission fails. Here are some examples of SMTP responses, from the SendGrid website (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 (this doesn’t mean your message has been opened…).
• 421 – Your message was temporarily deferred by the recipient’s 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 recipient’s mailbox was unavailable. This may occur if the mailbox is locked or not routable.
• 451 – This response is sent when the message simply failed. Often, it’s not your fault but the fault of a remote server.
• 452 – This type of response is sent when there is insufficient system storage to send the message. Your message is deferred until space is cleared and it can be delivered.
• 550 – The message failed because the recipient’s mailbox is unavailable or because the recipient’s server rejected your message.
• 551 – The mailbox to which your message was sent 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 mailbox name you sent it to does not exist.