Before we dive into SMTP Relay, it is important to know how the SMTP protocol works. SMTP is an acronym for Simple Mail Transfer Protocol. Most of Internet service providers nowadays use this protocol to send email. Email clients, also known as Mail User Agents (MUA), utilize this protocol and act as an SMTP client to distribute email messages to the recipients. When a MUA sends an email messages, it connects to the configured SMTP server and communicates to it using the SMTP protocol.
Internet mail works pretty much like our postal mail. When you wish to send a letter or a package via snail mail, you put the letter inside an envelope, write the recipient's as well as your return address and drop it off at your local post office. The local post office figures out the final destination of the package and sends it to the appropriate post office in the recipient's town. One important factor to notice here is that if both sender and recipient are in the same town no other post office gets involved.
Electronic mail works pretty much the same. SMTP servers act as local post offices. When a user wishes to send an email, he or she sends it to the SMTP server, which then forwards it to the recipient's SMTP server. Rather than street address and apartment numbers, electronic mail recipients are identified by unique email addresses. Every SMTP server is configured to handle one or more domain names. Analogous to snail mail, if both sender and recipient are in the same domain no other SMTP server gets involved. Following characteristics are common between snail mail and electronic mail.