How to understand the SMTP Server Response Codes? Well, it comes out that all the SMTP Server Response Codes are standard. In other words, SMTP servers always respond to requests/commands using the very same defined set of codes. The SMTP Server Response Codes are part of the SMTP protocol, the set of rules used to send emails. As a reminder, an SMTP server is the type of server you use to send emails. SMTP is also the name of the protocol that defines the delivery of emails over the internet. Sending a message actually consists of connecting to an SMTP …