SMTP Server Response Codes: What Do They Mean? Warren Duff March 12, 2014 Technical // SUMMARIES ?> Sending email involves multiple computers communicating with each other through a process call Simple Message Transfer Protocol (SMTP). SendGrid functions as an SMTP relay service to send billions of marketing and transactional emails every month! Within the SMTP process, messages are relayed from one computer to another via SMTP servers. SMTP servers send response codes that indicate what has happened to messages. While messages are usually sent without issue, different server response codes can indicate various things. Below are a handful of SMTP server response codes that we’ve compiled that you might see when sending email: 250 – This SMTP server response simply means everything went well and your message was delivered to the recipient server. 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. 554 – This is a very vague message failure response that can refer to any number of problems either on your end or with the recipient server. There are a variety of issues that can trigger these SMTP server responses, and you’re not expected to know how to handle all of them. SendGrid’s amazing support staff is here to help you figure out what went wrong and what you can do to correct it. To learn more about these and other SMTP server response, read our Knowledge Base article Common SMTP Server Responses. The article provides examples of all these responses as well as offers suggestions for correcting any problems.