The Simple Mail Transfer Protocol (SMTP) is a programme that is used to send, receive, and relay outgoing emails between senders and receivers. When an email is sent, it is transferred from one server to another via the internet using SMTP. In layman's terms, an SMTP email is simply an email sent through the SMTP server.
If the SMTP server is used to send emails, then SMTP relay can be defined as the process of transferring an email from one server to another. It is primarily used to deliver emails from one domain to another that is not the user's domain. The SMTP relay service can be used to resolve a variety of issues, including email deliverability and IP blacklisting.